linsysfs(4): Keep Linux compatible sysfs the same as Ubuntu

By checking Ubuntu, there is no `/sys/subsystem' in sysfs. To compatible
with Ubuntu, delete the 'subsystem' creation in Linux compatible module.

On the other hand, the sysfs `/sys/subsystem' cause failure for some
Linux udev cases. In Linux udev source code, there is a function named
`scan_devices_all', and it will scan `/sys/subsystem' if it is existed,
but now there are nothing in /sys/subsystem `, and it returns empty
to cause some use cases failed.

Reviewed by:		dchagin
Differential Revision:	https://reviews.freebsd.org/D38885
MFC after:		1 month
XMFC with:		ifAPI

sys/compat/linsysfs/linsysfs.c

@@ -535,9 +535,6 @@ linsysfs_init(PFS_INIT_ARGS)
 	/* /sys/kernel/debug, mountpoint for lindebugfs. */
 	pfs_create_dir(kernel, "debug", NULL, NULL, NULL, 0);
 
-	/* /sys/subsystem/... */
-	dir = pfs_create_dir(root, "subsystem", NULL, NULL, NULL, 0);
-
 	linsysfs_net_init();
 
 	return (0);