po: Remove a comment that is not valid any more since 2002-01-07.

po: Remove a comment that is not valid any more since 2002-01-07.

The comment "Target 'all' must not depend on target '$(DOMAIN).pot-update'"
does not hold any more:
  all <- all-yes <- $(srcdir)/stamp-po <- $(srcdir)/$(DOMAIN).pot
and the latter rule executes
  $(MAKE) $(DOMAIN).pot-update

* gettext-runtime/po/Makefile.in: Remove comment.
* gettext-tools/examples/po/Makefile.am: Likewise.
* gettext-tools/examples/*/po/Makefile.am: Likewise.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile: Likewise.