Fix default signature of remaining to align with method type

[bgamari]

This allows bytes to compile with GHC 8.2.

[bgamari]

Pinging @RyanGlScott. Also see GHC #13249.

[RyanGlScott]

FWIW, this:

default remaining :: (MonadTrans t, Monad n, MonadGet n, m ~ t n, Remaining m ~ Remaining n) => m (Remaining m)

Can be made even shorter:

```haskell
default remaining :: (MonadTrans t, MonadGet n, m ~ t n, Remaining m ~ Remaining n) => m (Remaining m)

That Monad n constraint is redundant, as it is implied by MonadGet n.