Remove `SYNAPSE_USE_FROZEN_DICTS` environment variable (#18123)

I got rid of the `SYNAPSE_USE_FROZEN_DICTS` environment variable because
it will be overridden by the Synapse worker apps anyway and if we want
to support `SYNAPSE_USE_FROZEN_DICTS`, it should be in
`synapse/config/server.py`. It's also not documented so I'm assuming no
one is using it anyway.

Spawning from looking at the frozen dict stuff during the review of
https://github.com/element-hq/synapse/pull/18103#discussion_r1935876168