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