targets.mk: Drop the rule to clean at every compile

Rationale: 1. Save time and energy.  2. Use the make tool for what
it's been made.

mk/targets.mk

@@ -88,8 +88,6 @@ local.mk:
 	$(info ======================================================)
 	-@$(MAKE_LOCAL_MK)
 
-all compile::
-	$(foreach dir, doc lisp, $(MAKE) -C $(dir) clean;)
 compile compile-dirty::
 	$(MAKE) -C lisp $@
 all clean-install::