topic: drop topicfilter usage from topicrepo.commitctx()

This block of code was introduced in 13313d0cab71, but the commit description
doesn't explain the exact intent of this code.

The fact that the result of other.commitctx() was discarded makes me think that
this block only existed to deal with amends, and it was only useful for code
that was removed earlier in this series.

The only code that's left in commitctx() is adding current topic to
workingcommitctx, and I don't think we need to adjust repo filter for that.
8 jobs for topic/default/old-topic-amend in 4 minutes and 43 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #243654

00:01:12

passed checks-py3 #243655

00:01:45

passed doc #243660

00:00:26

manual pytype #243653
allowed to fail manual
passed tests-py2-cext #243656

00:03:16

passed tests-py2-pure #243657

00:04:21

passed tests-py3-cext #243658

00:04:41

passed tests-py3-pure #243659

00:02:14