Cancel obsolete builds on reconfiguration

Cleanup running builds for jobs that are no longer defined on
reconfiguration.

Change-Id: I746a5ba8d9034ae846fd77080af2bfabc8aedf44
Remove double accounting of builds

The "building_jobs" dictionary on pipeline managers is no longer
necessary because the object data model is sufficient to find all
of the running builds.  Remove the unnecessary second list.

Some of the protections that checked that builds were in this
dictionary are also no longer needed as it is not possible to
receive a result for a build that the running Zuul did not
request since the change to use Gearman.

Change-Id: I10de7edcf8356885f5f2d2f45d7014582bcbc3cf
Merge "Cancel obsolete builds on reconfiguration"
Merge "Remove double accounting of builds"