kbuild: Drop support for clean-rule

clean-rule has not been used since 94869f86 (kbuild: Accept absolute
paths in clean-files and introduce clean-dirs) ten years ago.
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/Makefile.clean

@@ -70,9 +70,6 @@ ifneq ($(strip $(__clean-files)),)
 endif
 ifneq ($(strip $(__clean-dirs)),)
 	+$(call cmd,cleandir)
-endif
-ifneq ($(strip $(clean-rule)),)
-	+$(clean-rule)
 endif
 	@: