Remove redundant contexts from Foldable methods

New `Foldable` methods accidentally had `Foldable` contexts, which led
to type roles being assigned incorrectly and preventing GND from
deriving `Foldable` instances. Removing those fixes #9761.

Moreover, this patch takes advantage of this fix by deriving
`Foldable` (and `Eq`) for `UniqFM`.

Differential Revision: https://phabricator.haskell.org/D425