[FIX] orm: ordering on >1000 records

When reading over IN_MAX (currently 1000) records, the select is slip into
subsets of IN_MAX records. However the list of ids was split using set() method
which removes order that may have been pass. The subsets are ordered using
_order attribute but the subsets were not ordered between themself.
This is an issue in case of browsing a o2m field with more than 1000 lines as it
will return sorted blocked but the order of the blocks is the order of the
contained ids (e.g. split(2, [5, 4, 3, 2, 1]) -> [[2,1], [4,3], [5]]).

Removes the set() to make sure the order of the given ids is preserved.
opw 616070, linked to #439