Revert "Update po Makefile to regenerate .po file on .pot update"

Not necessary, as we can configure Weblate to automatically update its
.po files on .pot update.

This reverts commit 59004e8.

Author: jocelynj

web/po/Makefile

@@ -7,7 +7,7 @@ MO:=$(patsubst %.po,mo/%/LC_MESSAGES/osmose-frontend.mo,$(PO))
 
 .PRECIOUS: $(PO) $(MO)
 
-all: osmose-frontend.pot $(PO) mo
+all: osmose-frontend.pot mo
 
 ../node_modules/easygettext/src/extract-cli.js:
 	$(error 'easygettext missing. Please install node dependencies with `npm install`')
@@ -34,9 +34,6 @@ osmose-frontend.pot: python.pot vue.pot
 	mv $@.tmp $@
 	rm $@.nofiles
 
-%.po: osmose-frontend.pot
-	msgmerge --previous --update $@ osmose-frontend.pot
-
 mo: $(MO)
 
 mo/%/LC_MESSAGES/osmose-frontend.mo: %.po