incorrect field in query in flag_get_flag_counts()

In flag_get_flag_counts(), the query adds a field that doesn't exist on the {flag_counts} table.

This doesn't cause an error, because the query is made into a count query, and doing that causes the query builder to throw away the fields before it executes the query.

However, it's confusing to developers trying to read the code to have a field that makes no sense!