Don't require -Wextra goto-gcc flag for warnings to be treated as errors

The new version is not completely equivalent to how it would work in
gcc. In gcc, when both -Werror and -Wno-error are given (possibly
multiple times), the flag that was given last "wins". However we
currently don't have a way of knowing the order of options in cmdlinet.

src/goto-cc/gcc_mode.cpp

@@ -364,7 +364,6 @@ int gcc_modet::doit()
   compilet compiler(cmdline,
                     gcc_message_handler,
                     cmdline.isset("Werror") &&
-                    cmdline.isset("Wextra") &&
                     !cmdline.isset("Wno-error"));
 
   if(cmdline.isset("version"))