Remove `contractive_ne` and `contractive_proper` as instances.

Instead, make them `Hint Immediate` so they are only used at leaves.
Status Job ID Name Coverage
  Build
passed #72115
fp
build-coq.8.10.2

00:08:41

passed #72114
fp-timing
build-coq.8.11.1

00:07:02

passed #72113
fp
build-coq.8.11.dev

00:15:22

passed #72116
fp
build-coq.8.9.1

00:08:11