Remove `BootstrapMixin`

[jeremystretch]

Proposed Changes

Ditch the BootstrapMixin class that's currently used to facilitate custom form field styling. Now that FORM_RENDERER has been configured to look for custom templates (see #13064), the functionality provided by this class can be offloaded to custom templates.

Justification

This change will remove a lot of cruft from existing form classes.