Proactively remove support for shared vars per the discussions in issue
https://b.elv.sh/1222 and the IM channels. This feature is not used
by Elvish itself and is not believed to be used by any Elvish users.
Removing this (presumptively) unused feature makes it simpler to replace
the existing BoltDB store of interactive command/location history with a
simpler flat file implementation (such as used by Bash and Fish). Also,
eliminating shared var support makes it explicit that solving issue #1222
doesn't need to deal with that feature.
Related #1222
