mdmonitor: Abandon custom configuration files

mdmonitor: Abandon custom configuration files

Operating system vendors are customizing mdmonitor service beacause
the default form is not satifying for them (expect SUSE). As a result,
support is complicated (maintainers have to check the system) and man page
is not detailed.

I propose to abandon custom configuration files via sysconfig and keep
it inside mdadm.conf only.

Detailed comment in service for OSV maintainers is added to help with
transition.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>