staging: fieldbus: Delete unused driver

staging: fieldbus: Delete unused driver

Sven Van Asbroeck contributed this driver in 2019.

The following reasons lead to the removal:
- This driver generates maintenance workload
- only 11 patches during the last 3 years. Part of the patches seem to be
  motivated because of maintenance (for example - remove deprecated
  function)
- Maintainer lost interest, last "Reviewed-by:" is May 2021
- no blog about usage of this driver

The staging subsystem is the way for drivers into the kernel - at current
speed and interest this is never going to happen. I think that fieldbus
is an interesting topic. But when almost nobody cares about this driver,
it does not make sense to keep it. Please consider that support will
remain for years in the longterm kernels.

Link: https://lore.kernel.org/linux-staging/96ae2b42-c0ce-4d9a-8933-eb874dc5589b@gmail.com/T/#u
Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/20241102191436.23177-1-philipp.g.hortmann@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>