Revert duplicate disable event enrolments functionality

Problem/Motivation

Comment fom Alexander
"In PR #2283 we introduced a new field 'field_event_enable_enrollment' but I can’t from the PR understand what the difference is with 'field_event_enroll?

One is “Enable event enrollment” and the other is “Allow users to enroll in the event” So it appears we duplicated functionality here?

There’s also 'field_enroll_method' to which we could also add a disabled state then we wouldn’t even need `field_event_enroll` anymore either.

Proposed resolution

- Revert PR 2283 entirely
- Include an update hook to remove the new field field_event_enable_enrollment.
- Then fix the original bug with the fields that already existed (field_event_enroll or field_enroll_method).
- Check compatibility with other features (e.g. anonymous enrolment, max event enrollment)