upas/common: delete library on 'mk clean'

libcommon.a$O doesn't end with a .a, so mk
doesn't know how to look inside it in order
to check if the files are up to date.

This means that when 'mk clean' is run,
libcommon.a$O looks up to date:

	% mk clean
	...
	% mk
	mk: 'default' is up to date

Deleting the library works around this problem.

sys/src/cmd/upas/common/mkfile

@@ -14,10 +14,7 @@ OFILES=\
 HFILES=common.h\
 	sys.h\
 
-UPDATE=\
-	mkfile\
-	${OFILES:%.$O=%.c}\
-	$HFILES\
+CLEANFILES=$LIB
 
 </sys/src/cmd/mklib