QtQA: Remove qt.branch for this repository

Since this repository doesnt have a sync.profile, the qt.branch
property does not make much sense.

Without it, the CI will just fallback to use qtbase#stable

Change-Id: I7796202f80513518580c40bd8f5eb70c7e5b71e0
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>