Don't read otp_secret_encryption_key from hardcoded path in models/user

Variable `Gitlab::Application.config.secret_key_base` is set in config/initializers/secret_token.rb. It's very bad practice to use hard-coded paths inside an application and really unnecessary in this case.

Mirror of https://github.com/gitlabhq/gitlabhq/pull/10311

See merge request !4044