Revert "sysupgrade: run only one instance at a time."

This reverts commit e96a9a9af8.

The change breaks sysupgrade through LuCI and two-stage sysupgrade on
NAND targets. There is also a mismatch of file paths in lock and unlock
operations.

This commit was apparently neither properly tested, nor reviewed, so
drop it for now.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/base-files/files/sbin/sysupgrade

@@ -47,20 +47,6 @@ while [ -n "$1" ]; do
 	shift;
 done
 
-# just one instance
-if  ! lock -n /tmp/sysupgrade.lock
-then
-	echo "Another instance of sysupgrade already running"
-	echo "If you are sure this is wrong remove file /tmp/sysupgrade.lock"
-	exit 1
-fi
-
-# make sure the lock is removed when done even when killed
-finish() {
-	lock -u /var/sysupgrade.lock
-}
-trap finish EXIT INT TERM
-
 export CONFFILES=/tmp/sysupgrade.conffiles
 export CONF_TAR=/tmp/sysupgrade.tgz