Remove unused code from `OAuth2TokenProvider`

import java.util.List;

import android.app.Activity;

import com.fsck.k9.mail.AuthenticationFailedException;

     */

    List<String> getAccounts();

    /**

     * Request API authorization. This is a foreground action that may produce a dialog to interact with.

     *

     * @param username

     *         Username

     * @param activity

     *         The responsible activity

     * @param callback

     *         A callback to process the asynchronous response

     */

    void authorizeApi(String username, Activity activity, OAuth2TokenProviderAuthCallback callback);

    /**

     * Fetch a token. No guarantees are provided for validity.

     */

     * Invalidating a token and then failure with a new token should be treated as a permanent failure.

     */

    void invalidateToken(String username);

    /**

     * Provides an asynchronous response to an

     * {@link OAuth2TokenProvider#authorizeApi(String, Activity, OAuth2TokenProviderAuthCallback)} request.

     */

    interface OAuth2TokenProviderAuthCallback {

        void success();

        void failure(AuthorizationException e);

    }

}

import java.net.UnknownHostException;

import java.util.List;

import android.app.Activity;

import com.fsck.k9.mail.AuthType;

import com.fsck.k9.mail.AuthenticationFailedException;

import com.fsck.k9.mail.CertificateValidationException;

                throw new UnsupportedOperationException();

            }

            @Override

            public void authorizeApi(String username, Activity activity, OAuth2TokenProviderAuthCallback callback) {

                throw new UnsupportedOperationException();

            }

        };

    }

}