Clean-up Brazilian Portuguese tree of translation files - remove invalid files.

It seems the last massive creation of missing files created too much
invalid files. It means these files wouldn't be not recognized by
Damned Lies, which means translators wouldn't find them in the webUI
and therefore wouldn't be transtated. It also means the translation
of these invalid files could be rendered not valid in the output of
GIMP Manual (not sure about that, though).

Another issue from this last massive creation is that some valid
files were created incomplete. By valid I mean that will be package
and translated, but some strings were "missing". Actually, these
"missing" strings went to invalid files.

So, this commit remove these invalid files, cleaning up po/pt_BR
folder. However, it is still needed to fix the valid files (which
will be done in next commit).
Clean-up Brazilian Portuguese tree of translation files - fix valid files.

This commit fixes the valid files that had some of their strings missing.
As explained in the last commit, these "missing" went to invalid files
in a massive creation of files.

Basically what was done with these files:
- Merged existing PO with the POT available in D-L - I used gmirr.sh to
  get POT files from https://l10n.gnome.org/POT/gimp-help-2.master/docs/

- Applied a default headers for all files, containing the basic lines in
  the begining (description of the file, copyright, license, authors) and
  some default headers (project id, language team email, plural-form...);

- Some correction on the translator-credits, in order to maintain the
  usual format of this string (name <email>, year);