nls: Remove the doc-po-update target.

As discussed with Julien Lepiller on IRC, updating the PO files is not useful.
This change removes the target and associated rules that allowed updating
them.  This has the benefit of not having the PO files updated automatically
when generating the .pot files, which was causing spurious changes under the
po sub-directory.

* po/doc/local.mk (make-update-po-files-rule): Remove function and its uses.
(doc-po-update): Remove target.

po/doc/local.mk

@@ -47,20 +47,6 @@ POT_OPTIONS = \
 	--copyright-holder "the authors of Guix (msgids)" 		\
 	--msgid-bugs-address "bug-guix@gnu.org"
 
-# make-update-po-files-rule PO_FILES POT_FILE_INPUT
-define make-update-po-files-rule
-$(1): $(2)
-	@if ! [ -f "$$@" ]; then \
-	    echo "File $$po_file does not exist.  If you are a translator, \
-you can create it with 'msginit'." 1>&2; \
-	    exit 1; \
-	fi && \
-	lang=$$$$(echo $$@ | $(SED) -E 's|.*\.([^.]*)\.po$$$$|\1|') && \
-	echo $(MSGMERGE_UPDATE) $(MSGMERGE_OPTIONS) \
-	  --lang=$$$${lang} "$$@" "$$<" && \
-	$(MSGMERGE_UPDATE) $(MSGMERGE_OPTIONS) --lang=$$$${lang} "$$@" "$$<"
-endef
-
 %D%/%.pot: $(srcdir)/doc/%.texi
 	$(AM_V_PO4A)$(PO4A_UPDATEPO) -M UTF-8 -f texinfo -m "$<" \
 	   -p "$@" $(POT_OPTIONS) && \
@@ -69,10 +55,5 @@ endef
 %D%/guix-manual.pot: %D%/guix.pot %D%/contributing.pot
 	msgcat $^ > $@
 
-$(eval $(call make-update-po-files-rule,$(DOC_PO_FILES),%D%/guix-manual.pot))
-$(eval $(call make-update-po-files-rule,\
-  $(DOC_COOKBOOK_PO_FILES),%D%/guix-cookbook.pot))
-
 doc-pot-update: %D%/guix-manual.pot %D%/guix-cookbook.pot
-doc-po-update: $(DOC_PO_FILES) $(DOC_COOKBOOK_PO_FILES)
-.PHONY: doc-pot-update doc-po-update
+.PHONY: doc-pot-update