Revert "LibCore: Try to fix fuzzer build"

This reverts commit c5709c0aed02cf7c7c5cda914d9794ac5cab55e7.

Userland/Libraries/LibCore/AnonymousBuffer.cpp

@@ -24,10 +24,6 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#if defined(__linux__) && !defined(_GNU_SOURCE)
-#    define _GNU_SOURCE // For memfd_create, MFD_CLOEXEC
-#endif
-
 #include <LibCore/AnonymousBuffer.h>
 #include <LibIPC/Decoder.h>
 #include <LibIPC/Encoder.h>