Remove unneeded OAuth model wrappers

Remove OauthApplication, OauthAccessToken, and OauthAccessGrant model files.

These classes were empty wrappers around Doorkeeper models that inherited directly from ApplicationRecord, bypassing Doorkeeper's validations, associations, and behavior.

These wrapper classes were never referenced and posed a maintenance risk if used inadvertently in the future. Reference Doorkeeper's models directly instead (Doorkeeper::Application, Doorkeeper::AccessToken, and Doorkeeper::AccessGrant).

Author: aaronskiba