Use eldoc-mode in place of haskell-doc-mode

[gracjan]

In light of haskell/haskell-mode#1138 please do not use haskell-doc-mode. It doesn't work anymore and eldoc-mode is much better serving its function.

The line to change is here:

prelude/modules/prelude-haskell.el

Line 42 in 1f3ddb6

(haskell-doc-mode)

Thanks.

[bbatsov]

Thanks for the heads up!