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/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;