remove double declarataion spotted by cparser

[r26065]

ir/ir/irflag_t.h

@@ -107,6 +107,4 @@ get_node_verification_mode(void) {
 #define get_opt_optimize_class_casts()           _get_opt_optimize_class_casts()
 #define get_opt_suppress_downcast_optimization() _get_opt_suppress_downcast_optimization()
 
-extern void firm_init_flags(void);
-
 #endif