c-ares DNS resolver: remove unnecessary code in SRV callback

[apolcyn]

This call to grpc_ares_notify_on_event_locked from the SRV callback is unnecessary:

There are only a few places that on_srv_query_done_locked callback can be invoked from, and we already call notify_on_event_locked (updating polled file descriptors) after each of them:

1. Within the call to ares_query that initiated the SRV lookup

• notify_on_event_locked is called afterwards

2. When ares_process_fd is called from the c-ares backup poller.

• notify_on_event_locked is called afterwards

3. When ares_process_fd is called because an fd became readable.

• notify_on_event_locked is called afterwards

4. When ares_process_fd is called because an fd became writable.

• notify_on_event_locked is called afterwards

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Furthermore, this call to grpc_ares_notify_on_event_locked from the SRV callback was buggy, because in certain cases it could destroy the fd that generated the on_readable_locked callback it's being invoked from, which could then cause a use-after-free in here. So it looks like the direct cause of b/243676671.