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/LibGfx/Color.h

@@ -350,11 +350,6 @@ public:
         return m_value == other.m_value;
     }
 
-    constexpr bool operator!=(Color const& other) const
-    {
-        return m_value != other.m_value;
-    }
-
     String to_string() const;
     String to_string_without_alpha() const;
     static Optional<Color> from_string(StringView);

Userland/Libraries/LibGfx/Point.h

@@ -132,12 +132,6 @@ public:
         return x() == other.x() && y() == other.y();
     }
 
-    template<class U>
-    [[nodiscard]] bool operator!=(Point<U> const& other) const
-    {
-        return !(*this == other);
-    }
-
     [[nodiscard]] Point<T> operator+(Point<T> const& other) const { return { m_x + other.m_x, m_y + other.m_y }; }
 
     Point<T>& operator+=(Point<T> const& other)

Userland/Libraries/LibGfx/Rect.h

@@ -477,12 +477,6 @@ public:
         return location() == other.location() && size() == other.size();
     }
 
-    template<class U>
-    [[nodiscard]] bool operator!=(Rect<U> const& other) const
-    {
-        return !(*this == other);
-    }
-
     [[nodiscard]] Rect<T> operator*(T factor) const { return { m_location * factor, m_size * factor }; }
 
     Rect<T>& operator*=(T factor)

Userland/Libraries/LibGfx/Size.h

@@ -99,12 +99,6 @@ public:
         return width() == other.width() && height() == other.height();
     }
 
-    template<class U>
-    [[nodiscard]] constexpr bool operator!=(Size<U> const& other) const
-    {
-        return !(*this == other);
-    }
-
     constexpr Size<T>& operator-=(Size<T> const& other)
     {
         m_width -= other.m_width;