refactor: remove admission primitives

[tisonkun]

Summary

• remove FairShare, PriorityShare, and the admission module and Cargo feature
• remove the related documentation, integration tests, trait assertions, and benchmark
• record the FairShare removal in the changelog

After discussing this with @andylokandy, the author of the previously merged #134, we concluded that admission needs a redesign. This temporarily removes admission from the feature set while that redesign is worked out.

Validation

• cargo x lint
• cargo x check
• cargo x test --no-capture
• cargo x bench