kbuild: Buglet in Documentation/DocBook/Makefile

'make mrproper' incorrectly deletes *~-style editor backup files
from the top-level directory. This bug was introduced in 2.5.29
when the top-level Makefile was changed to remain in the top-level
directory when invoking DocBook's Makefile:

Documentation/DocBook/Makefile

@@ -154,7 +154,6 @@ LOG     :=      $(patsubst %.sgml, %.log, $(BOOKS))
 OUT     :=      $(patsubst %.sgml, %.out, $(BOOKS))
 
 clean:
-	@rm -f core *~
 	@rm -f $(BOOKS)
 	@rm -f $(DVI) $(AUX) $(TEX) $(LOG) $(OUT)
 	@rm -f $(PNG-parportbook) $(EPS-parportbook)