Simplify code. Remove redundant casts and checks.

From sources/javadoc:
hudson.model.AbstractProject#getTrigger(Class<T> clazz)
* Gets the specific trigger, or null if the propert is not configured for this job.
I agree that this simplifies the issue, and the code in GhprbTrigger is redundant and useless. The use of Optional.fromNullable here and the fact that getTrigger returns null if that particular class isn't listed as a trigger means that we don't need to check for the class or null when the value is returned. If this code were expected to change in behavior, then I'd agree the null and class checks should remain, but that code has been around since 2007.