Clean up use of 'rm' in Makefiles

Clean up use of 'rm' in Makefiles

Most of the ccan Makefiles use $(RM) to remove files.  However, 'rm' is
traditionally considered one of the few shell tools which can be used in
Makefiles without indirecting via a variable.

rm is also typically invoked with -f in Makefiles, so that it doesn't cause
errors if the files don't exist (because they haven't been built).  A
number of instances in ccan were missing this.

This corrects these warts.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>