Remove the unnecessary `is_nothrow_swappable` condition in `swap`.

[mpark]

Summary

Thanks to @lewissbaker who pointed out the unnecessary condition in
https://reviews.llvm.org/D81954#inline-756872. Since this codepath does not
make use of swap anyway (that codepath is a different branch), we can safely
remove this condition and produce better codegen when all types are nothrow
movable but are potentially-throwing swappable.

See codegen in https://gcc.godbolt.org/z/uDFZjz

libcxx/include/variant

void __swap(__impl& __that)  {
     } else {
       __impl* __lhs = this;
       __impl* __rhs = _VSTD::addressof(__that);
       if (__lhs->__move_nothrow() && !__rhs->__move_nothrow()) {
         _VSTD::swap(__lhs, __rhs);
       }
       __impl __tmp(_VSTD::move(*__rhs));
 #ifndef _LIBCPP_NO_EXCEPTIONS
-      if constexpr (__all<(is_nothrow_move_constructible_v<_Types> &&
-                           is_nothrow_swappable_v<_Types>)...>::value) {
+      if constexpr (__all<is_nothrow_move_constructible_v<_Types>...>::value) {
         this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
       } else {
         // EXTENSION: When the move construction of `__lhs` into `__rhs` throws
         // and `__tmp` is nothrow move constructible then we move `__tmp` back
         // into `__rhs` and provide the strong exception safety guarantee.
         try {
           this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
         } catch (...) {