accept_merge_request: explicitly remove source branch

A bug in python-gitlab > 2.7.0 [1] brought this up: In the past we
relied on the merge request's setting, whether the source branch should
be deleted or not after a merge. Since we're using the API's merge
function in a fully automated task, however, the branch should always be
deleted. Add the according argument explicitly.

[1]: https://github.com/python-gitlab/python-gitlab/issues/1750