Remove unused function
Remove unused function

Commit 0b6f15b8a247b48e72690f5593ff4daf29d4ae57 removed the only
callsite of memtuple_get_nullp_len(), so remove the function too
to avoid a compiler warning (and reduce cruft).

Reviewed-by: Tang Pengzhou <ebfb6fa79a634ed64a59bdd78edfa125b950b4d8@pivotal.io>
Reviewed-by: Ashwin Agrawal <924bf3437ad315c1c74f9c41d83153600b05f548@pivotal.io>

jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb