Remove thunderbirdPlus add-on from translation framework

[CyrilleB79]

This is a request on behalf of its author (@RPTools-org) to remove thunderbirdPlus add-on from translation framework.

This for the following reasons:

• the version in the SVN repository matches "Thunderbird+ (G4, TB 102) sélectionné" add-on, which is last tested with NVDA 2024.1, whereas there is a "Thunderbird+G5 (TB 115 - 128ESR - 141+) sélectionné" which is up-to-date.
• The TB+G4 add-on is not supported anymore, since the TB version it applies to is not supported anymore. So there won't be any new release.
• The source strings for this add-on are erroneous: they are in French, but all non-basic-ASCII characters are dropped from those strings (e.g. "é", "à", etc.).
• Source strings are now in English in newer versions of the add-on, but the English strings have not been merged in SVN.
• The add-on has finally been translated outside the central SVN framework.

SVN translation framework is end-of-life. So for the new version of the TB+ add-on (TB+G5), I'd encourage its author @RPTools-org either to continue with a non-centralized translation workflow for now and, optionally, to integrate the new translation framework when it has proven to be stable, popular and running.