runfvp: drop --parameter, users can use -- --parameter

Now that arbitrary parameters can be passed to the FVP binary, runfvp
doesn't need to handle --parameter itself.

Anyone using runfvp --parameter should simply use runfvp -- --parameter.

Change-Id: I8a79200fe927c253308731e7e0cb228e53cd989a
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>