netmap: drop compatibility FreeBSD code

Netmap users on FreeBSD are not supposed to import code from the
github netmap repository anymore. They should use the code that
is available in the src repo. We can therefore drop the compatibility
code.

MFC after: 7 days

(cherry picked from commit 3da494d3246e30a19d2107c8bb8a293ffd5a9da6)