Vector: Remove translations for non-existent message 'vector-desc'

Follow-up to Id7bde76b.

It seems that l10n-bot will only remove them if anything else changes
in any of the messages (see e.g. change Id40eee57), and this message
key is conflicting with the one used by the old Vector extension.
Letting it stay here is at best confusing and at worst problematic.

Change-Id: I649e305a5eaa019b9afeb4fd9aa1cbe06259db66