Bug: Inferior-haskell: do not insert repeated prompts.

[aitenate]

The recent change #1806 appears to have introduced an bug with inferior-haskell-mode output. The GHCi output can either be modified or missing entirely. Below are example of what an M-x run-haskell created buffer looks like without and with the change:

fc1ea06

ghci> ghci> :{
f x
  | x == 0 = 1
  | x < 0  = 1
  | otherwise = 2 * f (x - 1)
:}
ghci| ghci| ghci| ghci| ghci| ghci> f 1
2
ghci> f 1
2
ghci> f 1
2
ghci> f 1
2
ghci> 

495fb96

Loaded GHCi configuration from /nix/stghci> :{
f x
  | x == 0 = 1
  | x < 0  = 1
  | otherwise = 2 * f (x - 1)
:}
ghci> f 1
ghci> f 1
2ghci> f 1
2
ghci> f 1
2
ghci> f 1
2
ghci> 

This also impacts the use of Org-babel source blocks, which for me, may of may not output some subset of the result with repeated executions of the same pure expression.

While not wholly solving the original issue :set prompt-cont "" does get rid of most of the additional prompts in the original issue.

[borgauf]

My Haskell Babel no longer has a healthy REPL. Running an initial Babel code block no longer produces the usual *haskell* REPL buffer. Telling it specifically in code block with, e.g., :session *myhaskellrepl* does start a REPL in new buffer, but it's a zombie that doesn't work. haskell-mode by itself works fine though...

[purcell]

I've reverted the related PR for now.