Remove implementation/handling of the hippo:skipindex mixin

The hippo:skipindex feature (introduced in REPO-745) has some undesirable side-effects (like no longer being able to lookup/search for specific nodes/nodetypes having this mixin).

In practice the usage/need for this feature has largely gone away (hippo:lockable and targeting:lock nodes having frequent lock refresh updates without need for indexing), because we no longer use hippo:lockable.

Therefore with v13 we will remove the implementation of this feature.
In a later (major) release we may also remove the mixin itself (for now it simply will be ignored).