contrib/flex: Drop local __dead2 patch

Upstream flex has added a yynoreturn macro, so this diff is no longer
needed. Partially reverts r181269.

Reviewed By:	jkim
Differential Revision: https://reviews.freebsd.org/D29679

contrib/flex/src/flex.skl

@@ -216,12 +216,6 @@ m4_ifdef( [[M4_YY_TABLES_EXTERNAL]],
 
 /* First, we deal with  platform-specific or compiler-specific issues. */
 
-#if defined(__FreeBSD__)
-#include <sys/cdefs.h>
-#else
-#define	__dead2
-#endif
-
 /* begin standard C headers. */
 %if-c-only
 #include <stdio.h>
@@ -653,7 +647,7 @@ m4_ifdef( [[M4_YY_NOT_IN_HEADER]],
 static yy_state_type yy_get_previous_state ( M4_YY_PROTO_ONLY_ARG );
 static yy_state_type yy_try_NUL_trans ( yy_state_type current_state  M4_YY_PROTO_LAST_ARG);
 static int yy_get_next_buffer ( M4_YY_PROTO_ONLY_ARG );
-static void yynoreturn yy_fatal_error ( const char* msg M4_YY_PROTO_LAST_ARG ) __dead2;
+static void yynoreturn yy_fatal_error ( const char* msg M4_YY_PROTO_LAST_ARG );
 ]])
 
 %endif