Fix debugger warnings for actors

[RailKill]

Mostly just unused variables/parameters and unused return values. Try not to use ignore-warning comments. For unused variables/params, either remove them or put underscore in front of the name. For unused return values, it depends; for error codes, we can print them out, for things like move_and_slide(), the velocity should be recorded (assigned to variable) - if there is no need to record its velocity, then there's a big question whether or not it even needs to be a KinematicBody.