refactor: remove legacy fallback for mobile: getAppStrings

This change removes the legacy fallback mechanism when executing
`mobile: getAppStrings` in `appium/webdriver/extensions/applications.py`.
The `app_strings` method now exclusively uses the `execute_script`
approach.

Correspondingly, the `GET_APP_STRINGS` command registration in
`Applications._add_commands` and the command definition in
`MobileCommand` have also been removed.

Author: Dor-bl