scmutil: drop unreachable except clause
socket.error is a subclass of IOError, which we catch higher up. It
seems to have been this way since 020a896a5292 (dispatch: sort
exception handlers, 2009-01-12), so let's celebrate the 10 year
anniversary (a few days late) of it being wrong by deleting it.
Differential Revision: https://phab.mercurial-scm.org/D5626