colexec: remove a redundant assertion

This commit removes a redundant assertion from the sort code where we
previously enforced that we had at least one partition to sort (which is
always the case). The assertion seems useless since the code works just
fine even if zero partitions are given (which will never be the case
since the sort operator short-circuits on a zero-length batch).

Release justification: low-risk cleanup.

Release note: None