Revert "ALSA: seq: Protect in-kernel ioctl calls with mutex"

This reverts commit feb68902.

The fix attempt was incorrect, leading to the mutex deadlock through
the close of OSS sequencer client.  The proper fix needs more
consideration, so let's revert it now.

Fixes: feb68902 ("ALSA: seq: Protect in-kernel ioctl calls with mutex")
Reported-by:  <syzbot+47ded6c0f23016cde310@syzkaller.appspotmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>