Use default entity behavior for agent in Activity
Use default entity behavior for agent in Activity

Since `Krikri::Activity` never alters the `SoftwareAgent`'s entity
behavior after initialization, there's no need to initialize it at
all. Instead, we simply use the default `EntityBehavior` for the
`SoftwareAgent` class.

dpla/KriKri,dpla/KriKri,dpla/KriKri,dpla/KriKri