Remove Router::has_route

[plafer]

We currently use has_route() in ExecutionContext::execute(), but has_route() typically is implemented as

fn has_route(&self, module_id: &ModuleId) -> bool {
    self.get_route(module_id).is_some()
}

However, get_route() (immutable) success doesn't imply get_route_mut() (mutable) will succeed. Our has_route() call is redundant with the later calls to get_route() anyways.

[PanGan21]

Hey @plafer i hope pr #740 is what is meaning to be done in this issue. If I misunderstood it please let me know.