Remove a sanity check in the exclusion-constraint code that prevented users

from defining non-self-conflicting constraints.

Jeff Davis

Note: I (tgl) objected to removing this check in 9.0 on the grounds that it
was an important sanity check in new, poorly tested code.  However, it should
be all right to remove it for 9.1, since we'll get field testing from the
9.0 branch.