Remove the waiting time when restart test DoH servers

We used to wait for 100 milliseconds before restart test DoH
servers in order to avoid binding a socket address that hasn't
be released. With this change, test DoH servers can try to bind
the socket address again, so the 100-milliseconds waiting time
can be removed.

Bug: 215818810
Test: wrote and ran a test that calls doh.stopServer() and then
      doh.startServer() 5000 times
Change-Id: I89acfd0997beeca0eb53139919a56931b2e79986