Everywhere: Remove redundant inequality comparison operators

C++20 can automatically synthesize `operator!=` from `operator==`, so
there is no point in writing such functions by hand if all they do is
call through to `operator==`.

This fixes a compile error with compilers that implement P2468 (Clang
16 currently). This paper restores the C++17 behavior that if both
`T::operator==(U)` and `T::operator!=(U)` exist, `U == T` won't be
rewritten in reverse to call `T::operator==(U)`. Removing `!=` operators
makes the rewriting possible again.
See https://reviews.llvm.org/D134529#3853062

Userland/Libraries/LibWeb/CSS/Angle.h

@@ -43,11 +43,6 @@ public:
         return m_type == other.m_type && m_value == other.m_value;
     }
 
-    bool operator!=(Angle const& other) const
-    {
-        return !(*this == other);
-    }
-
 private:
     StringView unit_name() const;
 

Userland/Libraries/LibWeb/CSS/Display.h

@@ -35,8 +35,6 @@ public:
         VERIFY_NOT_REACHED();
     }
 
-    bool operator!=(Display const& other) const { return !(*this == other); }
-
     enum class Outside {
         Block,
         Inline,

Userland/Libraries/LibWeb/CSS/Frequency.h

@@ -40,11 +40,6 @@ public:
         return m_type == other.m_type && m_value == other.m_value;
     }
 
-    bool operator!=(Frequency const& other) const
-    {
-        return !(*this == other);
-    }
-
 private:
     StringView unit_name() const;
 

Userland/Libraries/LibWeb/CSS/Length.h

@@ -121,10 +121,6 @@ public:
     // We have a RefPtr<CalculatedStyleValue> member, but can't include the header StyleValue.h as it includes
     // this file already. To break the cyclic dependency, we must move all method definitions out.
     bool operator==(Length const& other) const;
-    bool operator!=(Length const& other) const
-    {
-        return !(*this == other);
-    }
 
     float relative_length_to_px(Gfx::IntRect const& viewport_rect, Gfx::FontPixelMetrics const& font_metrics, float font_size, float root_font_size) const;
 

Userland/Libraries/LibWeb/CSS/Percentage.h

@@ -37,7 +37,6 @@ public:
     }
 
     bool operator==(Percentage const& other) const { return m_value == other.m_value; }
-    bool operator!=(Percentage const& other) const { return !(*this == other); }
 
 private:
     float m_value;
@@ -147,7 +146,6 @@ public:
             return (m_value.template get<Percentage>() == other.m_value.template get<Percentage>());
         return (m_value.template get<T>() == other.m_value.template get<T>());
     }
-    bool operator!=(PercentageOr<T> const& other) const { return !(*this == other); }
 
 protected:
     bool is_t() const { return m_value.template has<T>(); }

Userland/Libraries/LibWeb/CSS/Resolution.h

@@ -32,11 +32,6 @@ public:
         return m_type == other.m_type && m_value == other.m_value;
     }
 
-    bool operator!=(Resolution const& other) const
-    {
-        return !(*this == other);
-    }
-
 private:
     StringView unit_name() const;
 

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -109,7 +109,6 @@ public:
     float line_height(Layout::Node const&) const;
 
     bool operator==(StyleProperties const&) const;
-    bool operator!=(StyleProperties const& other) const { return !(*this == other); }
 
     Optional<CSS::Position> position() const;
     Optional<int> z_index() const;

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -402,7 +402,6 @@ public:
     virtual String to_string() const = 0;
 
     bool operator==(StyleValue const& other) const { return equals(other); }
-    bool operator!=(StyleValue const& other) const { return !(*this == other); }
 
     virtual bool equals(StyleValue const& other) const = 0;
 

Userland/Libraries/LibWeb/CSS/Time.h

@@ -41,11 +41,6 @@ public:
         return m_type == other.m_type && m_value == other.m_value;
     }
 
-    bool operator!=(Time const& other) const
-    {
-        return !(*this == other);
-    }
-
 private:
     StringView unit_name() const;