Remove twophase for cx_Oracle 6.x
Support for two-phase transactions has been removed entirely for
cx_Oracle when version 6.0b1 or later of the DBAPI is in use. The two-
phase feature historically has never been usable under cx_Oracle 5.x in
any case, and cx_Oracle 6.x has removed the connection-level "twophase"
flag upon which this feature relied.
Change-Id: I2e8161cc2bc12f4845c9224cd483038112fe9734
Fixes: #3997
(cherry picked from commit
1b98ce355a60b7ad9898aeb412c21591bb438231)
(cherry picked from commit
41e92f85581589fd011f93c86b81439f71237433)