pgsql: Fix dblink_build_sql_insert() and related functions to handle

Log Message:
-----------
Fix dblink_build_sql_insert() and related functions to handle dropped
columns correctly.  In passing, get rid of some dead logic in the
underlying get_sql_insert() etc functions --- there is no caller that
will pass null value-arrays to them.

Per bug report from Robert Voinea.

Tags:
----
REL8_4_STABLE

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.82.2.7 -> r1.82.2.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.82.2.7&r2=1.82.2.8)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.25.2.2 -> r1.25.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.25.2.2&r2=1.25.2.3)
    pgsql/contrib/dblink/sql:
        dblink.sql (r1.21.2.1 -> r1.21.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/sql/dblink.sql?r1=1.21.2.1&r2=1.21.2.2)