Remove the Pike stable branch jobs from master gate

Tempest 21.0.0 is released now, which marks the end of support
for Pike release - https://review.opendev.org/#/c/670700/

We do not need to run the Pike stable job om master gate now.
Depends-On: https://review.opendev.org/#/c/670700/

Change-Id: I7db428c02bbe3ae8ecdc3679817cb20b7a8a63dd
1 file changed