refactor: remove legacy command fallbacks in Applications extension

Removes the deprecated `try/except` fallback mechanisms across all
application management methods in `Applications` extension class
(`background_app`, `is_app_installed`, `install_app`, `remove_app`,
`terminate_app`, `activate_app`, `query_app_state`).

These fallbacks were intended for older legacy Appium servers that did
not support the `mobile:` extension script commands. Now, the SDK simply
uses the `execute_script` approach with `mobile:` prefixes directly.

Additionally:
- Removes unused exception imports.
- Removes unused `CanRememberExtensionPresence` inheritance.
- Cleans up `_add_commands` as legacy endpoints are no longer needed.
- Drops obsolete command constants from local usage.

Author: Dor-bl