Relunctantly removing stackforge/solum from projects.txt

We can't use python-mistralclient because:
https://review.openstack.org/102738

Rejection of taht patch means that we can not merge our Mistral
related code without removing ourselves form projects.txt. Our
gate jobs will not pass unless all of our requirements are in
global-requirements.txt, so until python-mistralclient makes its
way in there, we can not be part of projects.txt.

Change-Id: I2c5fe5777e5a8ba6cd81a5f76a0c6e77dd2ff8f6
Merge "Relunctantly removing stackforge/solum from projects.txt"