Wpcomsh: Remove actions and filters related to the Masterbar module

[fgiannar]

This PR removes actions and filters related to the Masterbar module.
The later is being deprecated (see product discussion) and WoA sites already load all Masterbar functionality via the masterbar package and independently of the module.
Therefore having the module active/inactive is redundant for WoA sites.
No functionality should be affected as this PR is removing all actions and filters that are related to the masterbar module.
This will allow us to completely remove the module from the Jetpack plugin in an upcoming release.

Proposed changes:

• On init: Stop force-enabling the masterbar module
• jetpack_get_available_modules filter: Stop conditionally unsetting the masterbar module upon fetching the Jetpack available modules
• jetpack_modules_list_table_items filter: Stop unsetting the masterbar module upon fetching the old Jetpack module list

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

pfwV0U-3U-p2

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Note that enabling/disabling the Masterbar module has no impact anyways as of Masterbar: Remove feature inclusion on Jetpack self-hosted sites #38804
• Therefore lets simply ensure no masterbar related Fatals/Warnings on your WoA dev blog with the current branch when activating/deactivating the module (via cli or wp-admin/admin.php?page=jetpack_modules)

[github-actions]

Thank you for your PR!

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Wpcomsh plugin:

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[coder-karen]

Tests well, no fatals / warnings 👍