projectile-get-all-sub-projects: remove unused KNOWN-PROJECTS arg

Also remove most of its doc-string, because the claims it makes are
either untrue or if they are true the code does not actually do anything
about it.  If the latter is the case, then the doc-string should at
least not claim that the issue is being addressed.
Use magit-status-internal instead of magit-status

In the development version `magit-status' is now only intended for
interactive use.  The stable version provides a forward compatibility
definition of `magit-status-internal'.
Merge pull request #568 from tarsius/silentio

Quiet byte compiler