Bug 1158896 - prevent dead code from being generated in lower.py; r=bent

Coverity was complaining that we have things like:

    if ((!(actor))) {
        return false;
    }

    if ((!(actor))) {
        return false;
    }

in the generated code, as the second return will clearly never be hit.  To
address this, let's remove a redundant call to dtorPrologue.

ipc/ipdl/ipdl/lower.py

@@ -4972,7 +4972,6 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
         actorvar = actor.var()
         method = MethodDefn(self.makeDtorMethodDecl(md))
 
-        method.addstmts(self.dtorPrologue(actor.var()))
         method.addstmts(self.dtorPrologue(actorvar))
 
         msgvar, stmts = self.makeMessage(md, errfnSendDtor, actorvar)