cubicweb #2015260 fix "broken db commands" in migractions [validation pending]

migractions.py has a number of commands flagged with the comment "broken db commands". These must be either repaired or removed.

prioritynormal
typebug
done in3.14.3
load0.000
load left0.000
closed by#166ad5458ed8 [migractions] fix comment to avoid ambiguity. Closes #2015260