Revert "Restore the versioning of the installed *.mo files"

This broke builddir != srcdir builds in GNOME Continuous:
  Making all in po
  make[2]: Entering directory '/ostbuild/source/gegl/_build/po'
  make[2]: *** No rule to make target '/config.status', needed by
    'Makefile'.  Stop.
  make[2]: Leaving directory '/ostbuild/source/gegl/_build/po'
  Makefile:643: recipe for target 'all-recursive' failed
  make[1]: *** [all-recursive] Error 1
  make[1]: Leaving directory '/ostbuild/source/gegl/_build'
  Makefile:550: recipe for target 'all' failed
  make: *** [all] Error 2

This reverts commit dc19cf4f.

https://bugzilla.gnome.org/show_bug.cgi?id=789010