libstdc++: Remove std::type_info::operator!= for C++20

This function can be synthesized by the compiler now.

	* libsupc++/typeinfo (type_info::operator!=): Remove for C++20.

libstdc++-v3/ChangeLog

 2020-02-20  Jonathan Wakely  <jwakely@redhat.com>
 
+	* libsupc++/typeinfo (type_info::operator!=): Remove for C++20.
+
 	* include/std/thread (thread::id::operator<=>): Define for C++20.
 	* testsuite/30_threads/thread/id/70294.cc: Do not take addresses of
 	functions in namespace std.

libstdc++-v3/libsupc++/typeinfo

@@ -133,8 +133,11 @@ namespace std
     { return __name == __arg.__name; }
   #endif
 #endif
+
+#if __cpp_impl_three_way_comparison < 201907L
     bool operator!=(const type_info& __arg) const _GLIBCXX_NOEXCEPT
     { return !operator==(__arg); }
+#endif
 
 #if __cplusplus >= 201103L
     size_t hash_code() const noexcept