remove $KEYSTONE_TOKEN_HASH_ALGORITHM

Now that Keystone is back to UUID token defaults the use case for this
in devstack is extremely dubious, and it can be set through via
local.conf if anyone *really* cares.

Part of bp:devstack-nounset

Change-Id: I644b5b1579952959d253758b2a12b97d8a704657
Merge "remove $KEYSTONE_TOKEN_HASH_ALGORITHM"