Get rid of `haskell-indentation-indent-leftmost`

[gracjan]

Variable haskell-indentation-indent-leftmostcan be t or nil or both, and it applies in this case:

func = case a of
  ^      ^

When t indentation offers just the left position, when nil offers just the rightmost, when both offers both.

There are some general truths about this situation:

1. Truth #_1: Nobody changes the defaults.
2. Truth #_2: People are upset when their favorite indentation point is not on the list.
3. Truth #_3: People are kind of okay with clicking TAB until it falls in place as they want.
4. Truth #_4: Cohort of people wanting rightmost indentation is progressively falling of the right edge of the screen.
5. Truth #_5: Cohort of people that care about indentation is growing smaller. People want it to just work.
6. Truth #_6: Determinism (as in: single option) is a virtue that could help with Make indent-according-to-mode continue previous line indentation on empty lines #1265.

Current default is t and I do not think there is a single person on this planet that changed it. There were some that wanted rightmost so this set is not empty.

Note: the fundamental reason to remove this defcustom is that it changes the algorithm. We do not test non t versions, if we keep this defcustom we have to start testing it. This is contrary to other numerical settings that just change the offsets.

[gracjan]

Interestingly indentation handles if..then..else with the same code as case..of and let..in. Now we want both/leftmost semantics for case..of. We want rightmost for if..then..else. let..in has leftmost/both a this point. Relevant renderings:

func = if x then
  y else z
func = if x then
         y else z
func = let x = 5 in
  x
func = let x = 5 in
         x
func = case  x of
  x
func = case x of
         x

Note that if the second keyword (of, then, else, in) is at the beginning of the line it is aligned with the starting keyword (case, if, let) without any offsets:

func = if x
       then y else z
func = let x = 5
       in x
func = case  x 
       of x