narrow: update TODO.rst now that the narrowspec is in .hg/store
We no longer have the unfortunate wrappostshare() and
unsharenarrowspec() since 576eef1ab43d (narrow: move .hg/narrowspec to
.hg/store/narrowspec (BC), 2018-08-02).
Differential Revision: https://phab.mercurial-scm.org/D4902