obsexchange: delete dead code for old exchange protocol

Dead since 79a926b557f1 (compat: drop support for older exchange
protocol, 2017-10-25).
2 jobs for topic/default/dead-code in 11 minutes and 54 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed flake8 #46134

00:00:15

passed tests #46135

00:11:53