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 Name Job ID Coverage
  Build
passed build-coq.8.10.dev #46622
fp

00:07:44

passed build-coq.8.7.2 #46628
fp

00:02:46

passed build-coq.8.8.0 #46627
fp-timing

00:02:45

passed build-coq.8.8.1 #46626
fp

00:02:50

passed build-coq.8.8.2 #46625
fp

00:02:55

passed build-coq.8.9.0 #46624
fp-timing

00:03:12

passed build-coq.8.9.1 #46623
fp

00:03:36

passed build-coq.dev #46621
fp

00:09:54