DoH: Don't call handle_draining() more than once

Calling handle_draining() is effective only for the first time.
Subsequent calls are not necessary because all of the relative
channels are already closed.

Bug: 215818810
Test: cd packages/modules/DnsResolver && atest
Test: checked the log that "draining on network" is printed only once
Change-Id: Ifae32daff39fc86f40550005f35f544b449dd37e