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/Display.h

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