Remove/clean up messenger references

[layoutd]

Changes proposed in this Pull Request:

Closes #2724.

This PR removes (nearly) all Messenger-related code to complete the process of Messgenger deprecation.

Detailed test instructions:

1. Unit tests pass vendor/bin/phpunit (I get 3 skipped).
2. Plugin works as usual with no errors
   1. Activate/deactivate plugin.
   2. Connect to a Facebook page. Smoke Test
   3. Product sync works Smoke Test.
   4. New product sync works Smoke Test.
3. Test upgrade paths still work.
   1. Use a fresh WC install.
   2. Install Facebook for WooCommerce 1.11.4 and activate.
   3. Close any open browser windows, delete facebook directory, install Facebook for WooCommerce repo, swap to this branch, rebuild.
   4. Reopen /wp-admin/plugins.php and confirm current version 3.2.3 and no errors.
4. Specifically test Messenger-related upgrade path.
   1. Use fresh WC install.
   2. Checkout repo, switch to branch <3.2.0
   3. Build and activate plugin, connect to Facebook page. Enable Messenger at /wp-admin/admin.php?page=wc-facebook&tab=messenger
   4. Checkout PR branch.
   5. Visit plugins page, confirm v3.2.3 shows, and no errors in logs.
   6. Confirm all Messenger related options were removed correctly

   SELECT *  FROM `wp_options` WHERE `option_name` LIKE '%facebook%messenger%';

Changelog entry

Tweak - Fully remove Facebook Messenger code references.

[ibndawood]

Thank you, @layoutd. I followed your test instructions and the changes test well.