utils/wlgen: RTA: remove .get() operations

RTATask already contains all of the required information to create a
RT-app workload. Thus, transforming an RTATask instance into a
dict (which is what .get() is doing) is superfluous.

This means that the creation of an rt-app workload changes from

RTA() = rta(...)
rta.conf({'foo' : Periodic().get()})

to

RTA() = rta(...)
rta.conf({'foo' : Periodic()})