Inferior-haskell: do not insert repeated prompts.

[cnngimenez]

Hi!

I found that ob-haskell.el shows multiple prompts at any code block output. This was a little annoying for me, so I tried to fix it. Then, I realized it was from the inferior-haskell: It did not remove the prompts from the process output.
For example, when starting an inferior Haskell with M-x run-haskell and writing a multiple line function, the "*haskell*" buffer looks like this:

Loaded package environment from /home/poo/.ghc/x86_64-linux-9.0.2/environments/default
GHCi, version 9.0.2: https://www.haskell.org/ghc/  :? for help
ghci> ghci> :{
f x
  | x == 0 = 1
  | x < 0  = 1
  | otherwise = 2 * f (x - 1)
:}

ghci| ghci| ghci| ghci| ghci| ghci> ghci> 

Therefore, I wrote a function to remove all prompts except for the last one. All continuation prompts (ending with "| ") are completely removed too. After that, I added it to the hook comint-preoutput-filter-outputs buffer-locally. Now, the result in the "*haskell*" buffer is:

Loaded package environment from /home/poo/.ghc/x86_64-linux-9.0.2/environments/default
GHCi, version 9.0.2: https://www.haskell.org/ghc/  :? for help
ghci> :{
f x
  | x == 0 = 1
  | x < 0  = 1
  | otherwise = 2 * f (x - 1)
:}

ghci> 

I believe that this output is easier to read, and the ob-haskell.el (and maybe other libraries) process the buffer output without the need for more code.

Is it good to include this modification?
Does this patch requires any other change?
Cheers.

[purcell]

Looking back through a couple of PRs, since nobody actively maintains haskell-mode these days, and this seems worth merging — thanks!

[purcell]

Going to revert this due to #1867, but if you want to fix it up and try again, let me know.