narrow: remove hack to read narowspec from shared .hg directory
This was another leftover from 576eef1ab43d (narrow: move
.hg/narrowspec to .hg/store/narrowspec (BC), 2018-08-02), in addition
to 623081f2abc2 (narrow: remove hack to write narrowspec to shared .hg
directory, 2018-09-12).
Differential Revision: https://phab.mercurial-scm.org/D4692
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}