Revert "Update jenkins jobs to optionally allow trusty"

This reverts commit 46b8b8857dad86431652217b0edd616dbf5c7b0b.

This is necessary as there is a bug in the Jenkins gearman-plugin that
prevents node selection from working properly. Adding this will cause
jobs to run on trusty even if we don't want them to run there. Revert to
avoid this problem.

Change-Id: Ie8541cdb4fafe0db87e1785374a26539811e80f5