fix: usr: Do not set SO_INCOMING_CPU

We currently set SO_INCOMING_CPU incorrectly, and testing by Ondrej
shows that fixing the issue by setting affinities is worse than letting
the kernel schedule threads without constraints. So we should not set
SO_INCOMING_CPU anymore.

Closes #4936

Merge branch '4936-remove-so-incoming-cpu' into 'main'

See merge request isc-projects/bind9!9497