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.

docs/installation/debian_based_en.md

@@ -182,7 +182,6 @@ sudo cp /opt/pleroma/installation/pleroma.service /etc/systemd/system/pleroma.se
 ```
 
 * Edit the service file and make sure that all paths fit your installation
-* Check that `EnvironmentFile` contains the correct path to the env file. Or generate the env file: `sudo -Hu pleroma mix pleroma.release_env gen`
 * Enable and start `pleroma.service`:
 
 ```shell

docs/installation/otp_en.md

@@ -149,9 +149,6 @@ chown -R pleroma /etc/pleroma
 # Run the config generator
 su pleroma -s $SHELL -lc "./bin/pleroma_ctl instance gen --output /etc/pleroma/config.exs --output-psql /tmp/setup_db.psql"
 
-# Run the environment file generator.
-su pleroma -s $SHELL -lc "./bin/pleroma_ctl release_env gen"
-
 # Create the postgres database
 su postgres -s $SHELL -lc "psql -f /tmp/setup_db.psql"