Field: Update SwitchField's label behavior once design is finalized

[behowell]

We're still deciding how the SwitchField label prop should work. Need to update the implementation once we have a design.

[behowell]

After talking to Design, we decided that SwitchField should always use the field's label, and not provide access to the underlying Switch's label. This is the currently implemented behavior, although I published a PR #24876 to remove the labelPosition prop, since it now has no effect.