Quality: Drop legacy dissolve reason columns, remove manual migration

[realmayus]

Back in #853, we migrated the integer-based dissolveReason to dissolveReasonEnum.
Then, dissolveReasonEnum became obsolete when we allowed the user to select multiple dissolveReasons in #941.

If the following query can be trusted,

SELECT "id", "studentId", "pupilId", "dissolveReasons", "dissolveReasonEnum", "dissolvedBy"
FROM match
WHERE dissolved = true
  AND NOT (
    array_length("dissolveReasons", 1) = 1
    AND "dissolveReasons"[1] = "dissolveReasonEnum"
  );

we only have five matches where dissolveReasons != [dissolveReasonEnum]:

  id   | studentId | pupilId | dissolveReasons | dissolveReasonEnum | dissolvedBy 
-------+-----------+---------+-----------------+--------------------+-------------
 17184 |     17613 |   28983 | {noMoreTime}    | accountDeactivated | pupil
 17350 |     17809 |   18845 | {ghosted}       | accountDeactivated | pupil
 17334 |     17801 |   29619 | {noMoreTime}    | other              | student
 17085 |     17550 |   29304 | {noMoreTime}    | accountDeactivated | pupil
 17227 |     17751 |   17654 | {noMoreTime}    | other              | pupil
(5 rows)

While I'm not 100% sure (due to logs being deleted regularly), I believe these are reactivated matches?
We ended up not writing to dissolveReasonEnum anymore; we're just updating dissolveReasons nowadays.

I think we should be fine with dropping dissolveReason and dissolveReasonEnum, but I would appreciate another set of eyes on this.

[Jonasdoubleyou]

I am a bit surprised that your query returns no match with more than one dissolve reason?

[realmayus]

I am a bit surprised that your query returns no match with more than one dissolve reason?

@Jonasdoubleyou We currently only have the option to select multiple reasons in the backend, the frontend match dissolve dialog has not been adjusted yet

SELECT "id" from match where array_length("dissolveReasons", 1) > 1; returns 0 rows