Don't specify C standard

This is already set globally for all GN targets.

Change-Id: Ifd5ca3f38c281a4f2fe8667578911454d95e9886
diff --git a/BUILD.gn b/BUILD.gn
index 5b054a5..3c4ed84 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -46,7 +46,6 @@
     "OPENSSL_NO_ASM",
     "_XOPEN_SOURCE=700",
   ]
-  cflags_c = [ "-std=c11" ]
   configs = [ ":boringssl_config" ]
 }