Delete transients at plugin deactivation

[anghelalexandra]

Describe the bug Deactivating the plugin should clear temporary data, in this case the transient containing information for the App Themes tab.

To Reproduce Steps to reproduce the behavior:

1. Activate the plugin and go to the App Themes tab
2. Deactivate the plugin
3. Check the wp_options table from the database - the pwa_more_updates transient is not deleted.

Expected behavior Clear transient(s) when the plugin is deactivated.