Removed unreachable tasks from `Master::Framework` on agent GC.
Removed unreachable tasks from `Master::Framework` on agent GC.

Unreachable tasks are stored in `Slaves` and `Framework` structs of
the master, but they were only being removed from the former when
an unreachable agent is GCed from the registry. This patch fixes it
so that the latter is also cleaned up.

Review: https://reviews.apache.org/r/69908

abudnik/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,chhsia0/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos,reneploetz/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos