remove .pot target from Makefile.in

We want to manually make the .pot file, we shouldn't have anything
in the Makefile which will cause the .pot file to be rebuilt
because of dependencies.

install/po/Makefile.in

@@ -185,9 +185,6 @@ create-po: $(DOMAIN).pot
 	    fi; \
 	done
 
-$(DOMAIN).pot: $(POTFILES)
-	$(MAKE) update-pot
-
 update-po: update-pot
 	$(MAKE) all