Remove QtJSBackend_stable_Integration CI.

qtjsbackend#stable is not used anymore after qtdeclarative#dev
was merged into qtdeclarative#stable

Change-Id: Id07f015ad5b4fea2aaaa5bb4a934b0c7b9653c2f
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>