Remove description of "Fixes:" tag

The development process states that a "Fixes: APERTIS-<task_id>" tag
should be added to relevant commit messages. At some point we had
functionality in place to automatically add comments on the Phabricator
task when such tags were present. I don't believe that this
functionality is still in place, hence the description is missleading.

This is also not something that I have seen being used (a lot?) by core
maintainers. Since this is not common practice we should remove this
requirement, simplifying the documented process for new contributors.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>