A wrong comment about search_indexed_tlist_for_var

The comment of search_indexed_tlist_for_var says:

 * In debugging builds, we cross-check the varnullingrels of the subplan
 * output Var based on nrm_match.

However, this cross-check will also be performed in non-debug builds
ever since commit 867be9c07, which converts this check from Asserts to
test-and-elog.  The commit message there also says:

    Committed separately with the idea that eventually we'll revert
    this.  It might be awhile though.

I wonder if now is the time to revert it, since there have been no
related bugs reported for quite a while.  Otherwise I think we may need
to revise the comment of search_indexed_tlist_for_var to clarify that
the cross-check is not limited to debugging builds.

Please note that if we intend to revert commit 867be9c07, we need to
revert 69c430626 too.

Thanks
Richard