[libc++] Remove the ability to use braced-init for filesystem paths

According to my reading of http://eel.is/c++draft/filesystems#fs.class.path,
the Standard doesn't actually mention that this should work. Since other
implementations don't allow it, allowing it in libc++ is just setting a
portability trap.

Supersedes https://reviews.llvm.org/D89865.

Differential Revision: https://reviews.llvm.org/D95975

GitOrigin-RevId: 1700ea7302870b38df2cd048d9920368dc87453b
diff --git a/include/filesystem b/include/filesystem
index b11de54..8d0b0f5 100644
--- a/include/filesystem
+++ b/include/filesystem
@@ -973,8 +973,8 @@
     return *this;
   }
 
-  template <class = void>
-  _LIBCPP_INLINE_VISIBILITY path& operator=(string_type&& __s) noexcept {
+  _LIBCPP_INLINE_VISIBILITY
+  path& operator=(string_type&& __s) noexcept {
     __pn_ = _VSTD::move(__s);
     return *this;
   }