Eliminate ChildAdmissionController after dropping support for Kueue 0.6

[dgrove-oss]

The ChildAdmissionController was a hacky workaround for Kueue 0.6 not correctly recognizing external frameworks.

We contributed upstream enhancements (kubernetes-sigs/kueue#2059) that are included in Kueue 0.7 that make this workaround obsolete. Once we can drop support for Kueue 0.6 from the main branch, we should remove this controller and related configuration and documentation (including controller architecture description in webpages).

Need to wait for #161 to be merged to main before doing this.

[dgrove-oss]

#185 updates our controller architecture description for Kueue 0.7