Remove read_admin_dashboard custom admin ability

What does this MR do and why?

With !189148 (merged) users with any custom admin role can now access /admin (Admin Area Dashboard page) by default. Previously, this page was only accessible to non-admin users with read_admin_dashboard ability.

This MR removes the now unnecessary read_admin_dashboard custom admin ability.

References

Implements part of Allow access to /admin by default when user has... (#536133 - closed).

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