Changes

ipaccess: Convert BSC OML & RSL link to use stream_srv

This in turn allows running BSC Abis interfaces through io-uring
backend, which should provide performance improvements when used.

Drop ipaccess_fd_cb() since it has no real implementation anymore.
That was only used by ipaccess-config in osmo-bsc.git, which got its own
ipaccess specific e1_line driver recently.
The use of that API was a quick dirty hack at the time which should
have never happened.

Related: SYS#7063
Related: OS#5755
Related: OS#5756
Change-Id: Idf241c8f2fdb86d090d4132a9b316b7236402232