remove build dependency of libosmo-netif to libosmo-abis
The only reason fo this build dependency seems to be some 'lapd over datagram' example programs which are built but never used by anyone.
For code that merely serves as illustration to developers, we shouldn't mandate a 'strong' build dependency. The examples should not be built by default, and only if they are manually enabled at
--configure time, we should check for libosmo-abis presence via pkg-config.