use ZUUL_PROJECT in extensions

Removing git-mwext-in-root which forged the extension git repository
path using: /srv/ssd/zuul/git/mediawiki/extensions/{ext-name} . Instead
that will use the git definition from the default which uses
ZUUL_PROJECT environnement variable.

bug: 53470
Change-Id: Ie222feddee15b2e61a2ef5a9819072cacd8265ea