remove git-annex-shell compat code

remove git-annex-shell compat code

* Removed support for accessing git remotes that use versions of
  git-annex older than 6.20180312.
* git-annex-shell: Removed several commands that were only needed to
  support git-annex versions older than 6.20180312.
  (lockcontent, recvkey, sendkey, transferinfo, commit)

The P2P protocol was added in that version, and used ever since, so
this code was only needed for interop with older versions.

"git-annex-shell commit" is used by newer git-annex versions, though
unnecessarily so, because the p2pstdio command makes a single commit at
shutdown. Luckily, it was run with stderr and stdout sent to /dev/null,
and non-zero exit status or other exceptions are caught and ignored. So,
that was able to be removed from git-annex-shell too.

git-annex-shell inannex, recvkey, sendkey, and dropkey are still used by
gcrypt special remotes accessed over ssh, so those had to be kept.
It would probably be possible to convert that to using the P2P protocol,
but it would be another multi-year transition.

Some git-annex-shell fields were able to be removed. I hoped to remove
all of them, and the very concept of them, but unfortunately autoinit
is used by git-annex sync, and gcrypt uses remoteuuid.

The main win here is really in Remote.Git, removing piles of hairy fallback
code.

Sponsored-by: Luke Shumaker