Remove unnecessary unbind in LDAP search+bind mode

Remove unnecessary unbind in LDAP search+bind mode

Comments in src/backend/libpq/auth.c say: (after successfully finding
the final DN to check the user-supplied password against)

/* Unbind and disconnect from the LDAP server */

and later

/*
 * Need to re-initialize the LDAP connection, so that we can bind to
 * it with a different username.
 */

But the protocol actually permits multiple subsequent authentications
("binds") over a single connection.

So, it seems like the whole connection re-initialization thing was
just a confusion and can be safely removed, thus saving quite a few
network round-trips, especially for the case of ldaps/starttls.

Author: Anatoly Zaretsky <anatoly.zaretsky@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CALbq6kmJ-1+58df4B51ctPfTOSyPbY8Qi2=ct8oR=i4TamkUoQ@mail.gmail.com