Remove QtJSBackend_release_Integration CI.

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

Change-Id: I4b114d7c0c73a2205f937d6521ef41e83e1fa9ac
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>