core: Streamline RemoveExternalEvent canDoAction()

Streamlined and cleaned up RemoveExternalEventCommand's canDoAction()
method by using the early return pattern and overriding
setActionMessageParameters().

A nice side effect of this cleanup was do solve the possible
NullPointerExcetion FindBugs was complaining about, as the function
would not terminate if the event was null.

Change-Id: I3d2110e40ad17aea852e1002fe1de5e576dc8248
Signed-off-by: Allon Mureinik <amureini@redhat.com>