Removing a repetitive comment and refactoring the Application class in
Railties.

The comment on the +env_config+ method is repetitive, likely to get
outdated, and provides no useful information which cannot be gleamed
from the code. I'm therefore removing it. I'm also refactoring the check
for the presence of a secret_token in the configuration.
Merge pull request #10961 from wangjohn/changing_rails_env_config

Removing a repetitive comment and refactoring the Application class in Railties