tuple: drop extra restrictions for multikey index

Multikey index did not work properly with nullable root field in
tuple_raw_multikey_count(). Now it is fixed and corresponding
restrictions are dropped. This also means that we can drop implicit
nullability update for array/map fields and make all fields nullable
by default, as it was until e1d3fe8a
(tuple format: don't allow null where array/map is expected), as far as
default non-nullability itself doesn't solve any real problems while
providing confusing behavior (gh-5027).

Follow-up #5027
Closes #5192