absorb: use scmutil.cleanupnodes() also when obsmarkers are disabled

Authored by martinvonz.

Description

absorb: use scmutil.cleanupnodes() also when obsmarkers are disabled

A side-effect of this is the new warning in the test case, but the
warning is valid, so I don't think that's a problem.

Differential Revision: https://phab.mercurial-scm.org/D6035