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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif