Bug 1417264 - Remove target for autoconf.mk; r=nalexander

This target isn't referenced elsewhere in client.mk. It appears to be
dead.

When this target became orphaned, I don't know. I suspect at some point
some included .mk file attempted to "include autoconf.mk." But I'm not
sure what file that would have been or when it would have changed.

FWIW, autoconf.mk is generated by config.status, courtesy of it being
listed in CONFIGURE_SUBST_FILES.

MozReview-Commit-ID: AcPrihAou11

client.mk

@@ -176,11 +176,6 @@ $(OBJDIR)/Makefile: $(CONFIG_STATUS_DEPS)
 endif
 	@$(MAKE) -f $(TOPSRCDIR)/client.mk configure CREATE_MOZCONFIG_JSON=
 
-ifneq (,$(CONFIG_STATUS))
-$(OBJDIR)/config/autoconf.mk: $(TOPSRCDIR)/config/autoconf.mk.in
-	$(PYTHON) $(OBJDIR)/config.status -n --file=$(OBJDIR)/config/autoconf.mk
-endif
-
 ####################################
 # Build it