Remove the envify command

Instead of using `kamal envify` to generate the .env file, we now assume
that it will be in place for us.

Options in place of `kamal envify`:
1. Pre-generate the .env file
2. Create the env file in the `.pre-init` hook
3. Log into a secret store/check you are logged in in the pre-init hook
   Then use .dotenv command and variable substitution to interpolate the
   secrets.

test/integration/docker/deployer/app/.kamal/.env.erb

@@ -1,2 +0,0 @@
-SECRET_TOKEN='1234 with "中文"'
-SECRET_TAG='TAGME'

test/integration/docker/deployer/app_with_roles/.kamal/.env.erb

@@ -1 +0,0 @@
-SECRET_TOKEN='1234 with "中文"'