Rigify: Remove Legacy Mode

This patch removes the Legacy Mode checkbox from the addon preferences and all associated code.

Ability to convert old legacy metarigs is not removed.
LegacyRig wrapper class is not removed.
Various legacy-only functions and imports were not removed, with the reasoning that they are still used by [this unmaintained feature set](https://github.com/cessen/cessen_rigify_ext).

Initially discussed in T88711, let's put any further discussion here, since that thread has many topics.

Reviewed By: angavrilov

Differential Revision: https://developer.blender.org/D11516