Remove redundant "Minimal complete definition"-comments

Those manual descriptions in Haddock strings have become redundant since
Haddock gained the ability to print the minimal complete definition as
specified via `{-# MINIMAL #-}` annotation (or otherwise inferred by
GHC).

Moreover, this commit moves all `{-# MINIMAL #-}` annotations in `base`
to the start of the respective `class` definitions, as this is more
readable and matches more closely the way Haddock renders that
information.