lfs: don't add extension to hgrc after clone or share (BC)
Now that repository loading in core supports automatically loading
the lfs extension when the "lfs" requirement is present, we no
longer need to update the .hg/hgrc of newly-created repos to load
the lfs extension!
I'm marking this as BC because it is a change in behavior. But users
should not notice unless they create an LFS repo with new Mercurial
and then attempt to use it with an old versions that doesn't support
automatic extension loading.
Differential Revision: https://phab.mercurial-scm.org/D4712
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