Remove usage of _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY.

This macro is internal to libc++ and was removed by
https://github.com/llvm/llvm-project/commit/c6f3b7bcd0596d30f8dabecdfb9e44f9a07b6e4c

Abseil clients lacking this support should send a PR to add a
supported platform specific condition to the preprocessor to disable
it.

https://github.com/llvm/llvm-project/pull/89178#issuecomment-2075009955
suggests the feature test macros like __cpp_lib_filesystem now work.

PiperOrigin-RevId: 695454966
Change-Id: I9d4f710f422b9a2351d2780c219e8804f4162c07

absl/hash/internal/hash.h

@@ -69,8 +69,7 @@
 #include "absl/types/variant.h"
 #include "absl/utility/utility.h"
 
-#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
-    !defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
+#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L
 #include <filesystem>  // NOLINT
 #endif
 
@@ -605,7 +604,6 @@ H AbslHashValue(H hash_state, std::basic_string_view<Char> str) {
 #endif  // ABSL_HAVE_STD_STRING_VIEW
 
 #if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
-    !defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY) && \
     (!defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) ||        \
      __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000) &&       \
     (!defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) ||         \