Fix up Note [Bind free vars]

Move GHC-specific comments from Language.Haskell.Syntax.Binds to
GHC.Hs.Binds

It looks like the Note was deleted but there were actually two copies of
it. L.H.S.B no longer references it, and GHC.Hs.Binds keeps an updated
copy. (See #19252)

There are other duplicated notes -- they will be fixed in the next
commit