Compiler Attributes: ext4: remove local __nonstring definition

Commit 072ebb3b ("ext4: add nonstring annotations to ext4.h")
introduced a local definition of __nonstring to suppress some false
positives in gcc 8's -Wstringop-truncation.

Since now we support __nonstring for everyone, remove it.

Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # on top of v4.19-rc5, clang 7
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

fs/ext4/ext4.h

@@ -45,15 +45,6 @@
 
 #include <linux/compiler.h>
 
-/* Until this gets included into linux/compiler-gcc.h */
-#ifndef __nonstring
-#if defined(GCC_VERSION) && (GCC_VERSION >= 80000)
-#define __nonstring __attribute__((nonstring))
-#else
-#define __nonstring
-#endif
-#endif
-
 /*
  * The fourth extended filesystem constants/structures
  */