Remove “Close Buffer”.

The “Close Buffer” item was replaced by “Close Tab”, which was already added in commit 863bba4.
Replace “Close Buffer” by “Close Tab” in all the translations.
Remove duplicate “Close Tab” translation.

The translation removed is the translation of “Close Buffer”, while the other is the translation of “Close Tab”.
Merge pull request #21 from rastus-vernon/patch-1

Merge #21 Replace "Close Buffer" by "Close Tab" from @rastus-vernon