po: Remove obsolete Makefile rules for .mo files.
* gettext-runtime/po/Makefile.in.in (MSGFMT*): Remove variables.
(.SUFFIXES): Remove .mo.
(.po.mo): Remove rule.
(distclean): Don't remove *.mo files.
* gettext-tools/examples/hello-*/po/Makefile.am (.SUFFIXES): Remove .mo.
(.po.mo): Remove rule.
(DISTCLEANFILES): Remove *.mo.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile (.SUFFIXES):
Remove .mo.
* gettext-runtime/m4/po.m4 (AM_PO_SUBDIRS): Don't set MSGFMT_015.
