tests: remove unnecessary config from test-obsolete.t

mkcommit() is slightly different from what we have in common.sh, so it's left
as is.
8 jobs for topic/default/test-cleanup in 6 minutes and 23 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed checks-py2 #197680

00:02:02

passed checks-py3 #197681

00:02:03

passed doc #197686

00:01:17

manual pytype #197679
allowed to fail manual
passed tests-py2-cext #197682

00:05:23

passed tests-py2-pure #197683

00:04:26

passed tests-py3-cext #197684

00:03:33

passed tests-py3-pure #197685

00:05:06