kconfig/lxdialog: fix make mrproper

No Makefile in scripts/kconfig/lxdialog anymore, so do not
go there during make mrproper.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/kconfig/Makefile

@@ -135,7 +135,6 @@ endif
 
 clean-files	:= lkc_defs.h qconf.moc .tmp_qtcheck \
 		   .tmp_gtkcheck zconf.tab.c lex.zconf.c zconf.hash.c
-subdir- += lxdialog
 
 # Needed for systems without gettext
 KBUILD_HAVE_NLS := $(shell \