Remove previously deprecated features

[KevinOConnor]

This PR removes features that we have previously deprecated. I'll look to make this change in about a month. Details are in the PR:

Several deprecated features have been removed. Using "NTC 100K beta 3950" as a thermistor name has been removed (deprecated on 20211110). The SYNC_STEPPER_TO_EXTRUDER and SET_EXTRUDER_STEP_DISTANCE commands have been removed, and the extruder shared_heater config option has been removed (deprecated on 20220210). The bed_mesh relative_reference_index option has been removed (deprecated on 20230619).

-Kevin

[Arksine]

Looks good to me. Happy to see the hacks around the relative_reference_index go away.

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