Build process: Do not attempt to clean =contrib/=

mk/targets.mk: Remove "contrib" from 'CLEANDIRS'.

mk/targets.mk

@@ -5,7 +5,7 @@ DISTFILES_extra=  Makefile request-assign-future.txt contrib etc
 
 LISPDIRS      = lisp
 OTHERDIRS     = doc etc
-CLEANDIRS     = contrib testing mk
+CLEANDIRS     = testing mk
 SUBDIRS       = $(OTHERDIRS) $(LISPDIRS)
 INSTSUB       = $(SUBDIRS:%=install-%)
 ORG_MAKE_DOC ?= info html pdf