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.
Tones of small methods are not readable also. startJob() method in this variant is clean enough and fits into monitor without jumping between methods for 9 lines used only from one place. I'm also ok with current if() that is simple.