mdadm/systemd: remove KillMode=none from service file

mdadm/systemd: remove KillMode=none from service file

For mdadm's systemd configuration, current systemd KillMode is "none" in
following service files,
- mdadm-grow-continue@.service
- mdmon@.service

This "none" mode is strongly againsted by systemd developers (see man 5
systemd.kill for "KillMode=" section), and is considering to remove in
future systemd version.

As systemd developer explained in disuccsion, the systemd kill process
is,
1. send the signal specified by KillSignal= to the list of processes (if
   any), TERM is the default
2. wait until either the target of process(es) exit or a timeout expires
3. if the timeout expires send the signal specified by FinalKillSignal=,
   KILL is the default

For "control-group", all remaining processes will receive the SIGTERM
signal (by default) and if there are still processes after a period f
time, they will get the SIGKILL signal.

For "mixed", only the main process will receive the SIGTERM signal, and
if there are still processes after a period of time, all remaining
processes (including the main one) will receive the SIGKILL signal.

From the above comment, currently KillMode=control-group is a proper
kill mode. Since control-gropu is the default kill mode, the fix can be
simply removing KillMode=none line from the service file, then the
default mode will take effect.

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Benjamin Brunner <bbrunner@suse.com>
Cc: Franck Bui <fbui@suse.de>
Cc: Jes Sorensen <jes@trained-monkey.org>
Cc: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Xiao Ni <xni@redhat.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>