VCS: Clean up submit editor handling

It was not possible to simultaneously open two commit editors for
different version control systems, also there was no reason to scan all
open editors for the submit editor, since the plugins can just remember
the editor that they opened.

Change-Id: I1bea6ece3cd6faa1ecc0566bdd6f5fb10c816963
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>