Remove unused string resources

<?xml version="1.0" encoding="utf-8"?>

<resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">

    <string name="account_oauth_title">K-9 Mail</string>

    <string name="account_oauth_button_next">Next</string>

    <string name="account_oauth_button_back">Back</string>

    <string name="account_oauth_sign_in_title">Sign in with OAuth2</string>

    <string name="account_oauth_sign_in_description">We\'ll redirect you to your email provider to sign in. You need to grant the app access to your email account.</string>

    <string name="account_oauth_sign_in_button">Sign in</string>

    <string name="account_oauth_sign_in_with_google_button">Sign in with Google</string>