sql: remove one row limit in EXISTS subquery

According to ANSI, EXISTS is a predicate that tests a given subquery and
returns true if it returns more than 0 rows, false otherwise. However,
after 2a720d11, EXISTS worked correctly only if there were exactly 0
or 1 rows, and in all other cases it gave an error. This patch makes
EXITS work properly.

Closes #8676

NO_DOC=bugfix

(cherry picked from commit a5e498d1)