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

AK/StringView.h

@@ -245,11 +245,6 @@ public:
         return m_length == 1 && *m_characters == c;
     }
 
-    constexpr bool operator!=(char const* cstring) const
-    {
-        return !(*this == cstring);
-    }
-
 #ifndef KERNEL
     bool operator==(String const&) const;
 #endif