(core) Remove doc worker group elections

Summary:
A document can be marked in redis as belonging to a group, and then only
workers of that group serve it. A worker is told its group with
GRIST_WORKER_GROUP. There was a second way, where an operator stated in a hash
called groups how many workers each group wanted, and a worker starting without
a group claimed one of those places.

Nothing uses it. Grist Labs tells each Fargate pool its group, the groups hash
is unset, and nothing exercised the mechanism but its own test. It was broken
anyway. An elected worker was never told, so its availability went to that
group while the load it reported and the check for whether it was still
registered went to the group called default, where the load write updates
nothing and the check answers no for a worker that is running and serving.

The shunning recipe in the ecs readme now starts a pool the way the other pools
are started. Marking a document for a group is unchanged. A groups hash left in
redis is ignored from here, as are any elections-VERSION keys, and a worker map
built with a group of settings takes them one argument earlier.

Test Plan: updated tests

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D5052