Remove keystone-specs from the projects.txt

Keystone-specs is the only specs repository that was in this list
and it appears we aren't doing the same global-requirements gating
on the -specs repos.

This removes the keystone-specs repo until we determine if we want
to do global-requirements gating on -specs repos.

Change-Id: Ia1ca5e158b8c7080637fea1bff2dc252c6cc7ce1