Remove deprecated unlock LDAP membership feature

What does this MR do and why?

Related to #213311 (closed)

Removes ldap_settings_unlock_groups_by_owners feature flag and associated feature code. Feature was added years ago, never announced or default-enabled and we will not support the feature going forward.

There is also an associated database column which is ignored in this MR and will be removed in subsequent releases according to Avoiding downtime in migrations docs

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