branch: delete obsolete message about changing branch of obsolete commit
We now rely on rewriteutil.precheck() to check for divergence, so we
don't need the extra check in cmdutil.changebranch(). The former
check is a little less strict in that it allows you to rewrite a
commit without non-obsolete successors.
Differential Revision: https://phab.mercurial-scm.org/D10519