Minor cleanup of column-level priv fix

Minor cleanup of column-level priv fix

Commit 9406884af19e2620a14059e64d4eb6ab430ab328 cleaned up
column-privilege related leaks in various error-message paths, but ended
up including a few more things than it should have in the back branches.

Specifically, there's no need for the GetModifiedColumns macro in
execMain.c as 9.1 and older didn't include the row in check constraint
violations.  Further, the regression tests added to check those cases
aren't necessary.

This patch removes the GetModifiedColumns macro from execMain.c, removes
the comment which was added to trigger.c related to the duplicate macro
definition, and removes the check-constraint-related regression tests.

Pointed out by Robert.

Back-patched to 9.1 and 9.0.