Remove train testing from ironic-tempest-plugin

At this point, it is relatively safe to remove the
train jobs for ironic-tempest-plugin. The branches
are EOL, and we're unlikely to make any changes which
testing against Train, that can't be identified or
revealed in later community supported stable branches.

Change-Id: I069a9ec15d94871c7b610e130b13e9403110f728
1 file changed