Do not rely on macros with implicit vars

From is always the same value as in the Tag as both the local
`rpc` function and `gen:call/3` set it to `self()` in the caller.

With this change, we could even [change the `gen:call/3` tag
value to be a more meaningful value than `self()` here](https://github.com/erlang/otp/blob/maint/lib/stdlib/src/gen_event.erl#L190-L192). But I haven't included such change in this patch.