Feature request: Deregister relay environments from hook api

[junoatwork]

Currently, I can call __RELAY_DEVTOOLS_HOOK__.registerEnvironment(env) to add an environment to the devtools environment dropdown, but there is no corresponding way to remove (or deregister) an environment.

My motivation is for a sandbox tool where there may be many relay environments created or torn down within a page lifecycle. Currently, the dropdown is add-only, so after a short period of time we have no choice but to do a full-page reload.