po-*: move the Python goat exercise to the gimp30-std-plug-ins domain.
Since commit ecbc38f9, the Python goat exercise was tracked by the po-python/ Gettext files. This was an error as all its strings are common to all other goat exercise plug-ins, so it makes no sense to separate them (it only duplicates the work). Actually separating the translation domains by plug-in language was a bad idea from the start. Why do we care that a translatable string is written in this or that language? All our plug-ins should just share a common text domain. This way, we avoid useless duplication of common strings.