Coercible is now exported from GHC.Types (#8894)

so do not export it in GHC.Prim, and also have the pseudo-code for
GHC.Prim import GHC.Types, so that haddock is happy.
Remove support for "primclass"

This partly reverts commit e239753c349f925b576b72dc3445934cba8bcd50. Since
Coercible is exported via GHC.Types, so "primclass" is no longer
needed. The support for => in primops.pp is still required for coerce.