Revert "Makefile.extrawarn: re-enable -Wformat for clang"

This reverts commit 258fafcd.

The clang -Wformat warning is terminally broken, and the clang people
can't seem to get their act together.

This test program causes a warning with clang:

	#include <stdio.h>

	int main(int argc, char **argv)
	{
		printf("%hhu\n", 'a');
	}

resulting in

  t.c:5:19: warning: format specifies type 'unsigned char' but the argument has type 'int' [-Wformat]
          printf("%hhu\n", 'a');
                  ~~~~     ^~~
                  %d

and apparently clang people consider that a feature, because they don't
want to face the reality of how either C character constants, C
arithmetic, and C varargs functions work.

The rest of the world just shakes their head at that kind of
incompetence, and turns off -Wformat for clang again.

And no, the "you should use a pointless cast to shut this up" is not a
valid answer.  That warning should not exist in the first place, or at
least be optinal ...