requirements: Remove reference to swh-mirror-forge
The repository has been removed from swh-environment so no reference to it should be kept in requirements as the pip-swh-packages script fails otherwise.
(swh) ✔ ~/swh/swh-environment [master|✚ 1⚑ 5]
15:38 $ pip install $(./bin/pip-swh-packages --with-testing)
ERROR: swh-mirror-forge[testing] is not a valid editable requirement. It should either be a path to a local project or a VCS URL (beginning with svn+, git+, hg+, or bzr+).
Migrated from D2341 (view on Phabricator)