Remove AMP related data while uninstalling plugin.

[dhaval-parekh]

Summary

Fixes #3210

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[westonruter]

It looks like there is more data we can purge:

• The amp_css_transient_monitor_time_series option.
• The amp_customize_setting_modified_timestamps theme mod.

There may be some others but I think that's all the persistent data.

Finally, after deleting all persistent data, we can also purge a lot of transient data, specifically when not using an external object caching plugin (wp_using_ext_object_cache() returns false). We can do a straight SQL DELETE from the options table for all transients related to the plugin:

• All parsed CSS with the transient key beginning with amp-parsed-stylesheet-v.
• All cached image dimensions beginning with amp_img_.
• Validation errors URL count transient: amp_new_validation_error_urls_count.
• Validation error index count transient: amp_error_index_counts.
• (Optional) Validation errors generated during plugin activation with transient key: amp_plugin_activation_validation_errors. This has a 60 second expiration, so not important.
• (Optional) The themes from WordPress.org stored in the amp_themes_wporg transient. This has a 1-day expiration, so not important.
• (Optional) Image caching lock transients beginning with amp_lock_. This has a 60-second expiration, so not important.

This is particularly useful for users who have a bad experience with the AMP plugin when they have problems with the AMP plugin filling up their database with transients. By removing all of the transients upon uninstallation, their problem will be solved.

[westonruter]

I tested this by grabbing a SQL dump via wp db export --extended-insert=false before.sql and then I uninstalled and did another SQL dump via wp db export --extended-insert=false after.sql. Then I did diff before.sql after.sql and looked at the changes, all of which appear to be limited to the scope of what was intended to remove AMP data from the database.