Fixes #2496.
Previously we identified some endpoints that were receiving metrics
context data but would work equally well propagating stashed data
instead.
There's been some work to prepare those endpoints, e.g. in  804907a  we
ensured metrics context gets propagated through the account reset flow.
And in the content server, as of train 123 we no longer send metrics
context data to these endpoints. Ergo, it is now safe to remove it from
the payload schemata.
Note that this change presented us with a dilemma in the remote tests
for account reset. Some of the assertions there check that metrics
context data is set in the email headers correctly. That now depends on
memcached in order to work, but we were disabling memcached in the test
config. Rather than delete those assertions, which are valuable, I opted
to enable memcached instead. So memcached is now a prerequisite for the
tests to succeed in this repo.