rm mr_set_level, mr_annex_get commands (not used)

These were commands that I added in support of a shell-script
version of the ddr sync_group command, and are no longer used.
improved sync_group logging

In particular, added function to only log git command responses
if there's something there to log.
disable annex.sshcaching in sync_group function

SSH caching doesn't work on NTFS volumes in the version of
git-annex we're currently using.  It's disabled in all the other
commands that use annex, so disabling here too.