dist: systemd: use default KillMode

before this change, we specify the KillMode of the scylla-service
service unit explicitly to "process". according to
according to
https://www.freedesktop.org/software/systemd/man/latest/systemd.kill.html,

> If set to process, only the main process itself is killed (not recommended!).

and the document suggests use "control-group" over "process".
but scylla server is not a multi-process server, it is a multi-threaded
server. so it should not make any difference even if we switch to
the recommended "control-group".

in the light that we've been seeing "defunct" scylla process after
stopping the scylla service using systemd. we are wondering if we should
try to change the `KillMode` to "control-group", which is the default
value of this setting.

in this change, we just drop the setting so that the systemd stops the
service by stopping all processes in the control group of this unit
are stopped.

Fixes scylladb/scylladb#21507

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#21508

(cherry picked from commit 961a53f716)

Closes scylladb/scylladb#23177

dist/common/systemd/scylla-server.service

@@ -20,7 +20,6 @@ ExecStart=/usr/bin/scylla $SCYLLA_ARGS $SEASTAR_IO $DEV_MODE $CPUSET $MEM_CONF
 ExecStopPost=+/opt/scylladb/scripts/scylla_stop
 TimeoutStartSec=1y
 TimeoutStopSec=900
-KillMode=process
 Restart=on-abnormal
 User=scylla
 OOMScoreAdjust=-950