po files keep growing on `update-po`

[strk]

The make update-po rule leaves old (outdated) translations in the .po files, in lines starting with #~ prefix. This makes these files ever growing, so we need to decide on a trimming policy.

To make an example, the doc/po/ja/reference_processing.xml.po file has 3553 such lines, and only 3016 lines of used translations.

It would be useful to know if these translations are ever used by weblate, as if they are not we could just strip them in the make update-po rule itself.

[strk]

Trimming can be done with msgattrib:

msgattrib --no-obsolete -o po/ja/reference_processing.xml.po po/ja/reference_processing.xml.po

In addition to drop the obsoleted translations it also tend to reformat lines (line wrapping)

[strk]

Effects of removing the obsoleted strings from .po files:

251 files changed, 205 insertions(+), 300034 deletions(-)

[Sandro Santilli <strk@…>]

In 074189b/git:

Remove obsoleted translation strings on make update-pot

References #5454

[Sandro Santilli <strk@…>]

In 29caaad/git:

Remove obsoleted translations

Closes #5454