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