Move RemoveInheritedConstraint() call slightly earlier

Move RemoveInheritedConstraint() call slightly earlier

This change is harmless and does not affect the existing intended
operation.  It is necessary for a subsequent patch operation (NOT
ENFORCED foreign keys), where we may need to change the child
constraint to enforced.  In this case, we would create the necessary
triggers and queue the constraint for validation, so it is important
to remove any unnecessary constraints before proceeding.

This is a small change that could have been included in the previous
"split tryAttachPartitionForeignKey" refactoring patch (commit
1d26c2d2c4b), but was kept separate to highlight the changes.

Author: Amul Sul <amul.sul@enterprisedb.com>
Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b962c5AcYW9KUt_R_ER5qs3fUGbe4az-SP-vuwPS-w-AGA%40mail.gmail.com