Fix #3868: Remove operator!= to enable P2468R2 rewritten candidate synthesis

Under C++20 P2468R2, a hand-written operator!= suppresses the compiler's
rewritten-candidate synthesis for operator==, preventing heterogeneous
comparisons like `std::string s; json j; s == j;` from compiling.

Fix by removing the hand-written operator!=, allowing the compiler to
synthesize != as !(a==b) in all language modes (C++20 member functions
and pre-C++20 friend functions).

Behavior change: operator!= now returns !(a==b) unconditionally, including
for special values like NaN and discarded. This means:
- NaN != NaN now returns true (matches IEEE-754 semantics)
- discarded != x now returns true for any x (matches !(discarded == x))

This also fixes underlying defects in previously-working code:
- Restores direct == comparison for views vs json (reverts std::ranges::equal
  workaround added in PR #3950 to dodge this bug)
- Re-enables std::string == json comparisons (uncomments check in
  unit-constructor1.cpp)

Fixes: #3868, #3979

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-constructor1.cpp

@@ -278,7 +278,7 @@ TEST_CASE("constructors")
             const auto t = j.get<std::tuple<int, float, std::string>>();
             CHECK(std::get<0>(t) == j[0]);
             CHECK(std::get<1>(t) == j[1]);
-            // CHECK(std::get<2>(t) == j[2]); // commented out due to CI issue, see https://github.com/nlohmann/json/pull/3985 and https://github.com/nlohmann/json/issues/4025
+            CHECK(std::get<2>(t) == j[2]);
         }
 
         SECTION("std::tuple tie")