narrow: remove unrequired compat code for old versions of hg
As the comment says, that if is only required for servers having hg version 3.1
and 3.2. Any client connecting having hg 3.1 or 3.2 locally and trying to use
narrow should already be broken taking in account the changes which have been
done since narrow moved to core.
Differential Revision: https://phab.mercurial-scm.org/D6434