Remove `:>>` subclass instance declarations

There are not documented in
https://coq.inria.fr/refman/addendum/type-classes.html?highlight=class#coq:cmd.class,
and they don't have any advantage, so it's better to stop using them.
8 jobs for ci/robbert/no_forward_instances in 10 minutes and 4 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #46622
fp
build-coq.8.10.dev

00:07:44

passed #46628
fp
build-coq.8.7.2

00:02:46

passed #46627
fp-timing
build-coq.8.8.0

00:02:45

passed #46626
fp
build-coq.8.8.1

00:02:50

passed #46625
fp
build-coq.8.8.2

00:02:55

passed #46624
fp-timing
build-coq.8.9.0

00:03:12

passed #46623
fp
build-coq.8.9.1

00:03:36

passed #46621
fp
build-coq.dev

00:09:54