USB: core: Remove usbfs_mutex

Commit 4a2a8a2c ("usbfs: private mutex for open, release, and
remove") is now obsolete.  The commit was created back when we had
to handle both usbfs device nodes and the old usbdevfs filesystem
(/proc/bus/usb/), but usbdevfs no longer exists.

This means there's no longer any need to hold a mutex during two
separate removal operations (and thus during an entire notifier chain
call).  Furthermore, the one remaining remove/release pair doesn't
race with open thanks to the synchronization provided by the device
model core in bus_find_device().  Remove and release don't race with
each other because they both run with the device lock held.

The upshot is that usbfs_mutex isn't needed any more.  This patch
removes it entirely.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>