Remove useless code

Remove useless code

BuildDescForRelation() goes out of its way to fill in
->constr->has_not_null, but that value is not used for anything later,
so this code can all be removed.  Note that BuildDescForRelation()
doesn't make any effort to fill in the rest of ->constr, so there is
no claim that that structure is completely filled in.

Reviewed-by: Tomasz Rybak <tomasz.rybak@post.pl>
Discussion: https://www.postgresql.org/message-id/flat/a368248e-69e4-40be-9c07-6c3b5880b0a6@eisentraut.org