Remove release_env

Remove release_env

While taking a final look at instance.gen before releasing I noticed
that the release_env task outputs messages in broken english. Upon
further inspection it seems to have even more severe issues which, in
my opinion, warrant it's at least temporary removal:
- We do not explain what it actually does, anywhere. Neither the task
 docs nor instance.gen, nor installation instructions.
- It does not respect FHS on OTP releases (uses /opt/pleroma/config even
 though we store the config in /etc/pleroma/config.exs).
- It doesn't work on OTP releases, which is the main reason it exists.
Neither systemd nor openrc service files for OTP include it.
- It is not mentioned in install guides other than the ones for Debian
and OTP releases.