build: Remove generated files from version control.
This creates a bootstrapping issue, but it can be mitigated:
If a user wants to build GNU gettext on a platform which does not have the
GNU gettext programs installed, they first need to build and install a tarball
of GNU gettext; then only they can build GNU gettext from the git repository.
* gettext-runtime/po/gettext-runtime.pot: Remove file.
* gettext-tools/po/gettext-tools.pot: Remove file.
* gettext-tools/examples/po/gettext-examples.pot: Remove file.
