userns: Allow the usernamespace support to build after the removal of usbfs

The user namespace code has an explicit "depends on USB_DEVICEFS = n"
dependency to prevent building code that is not yet user namespace safe. With
the removal of usbfs from the kernel it is now impossible to satisfy the
USB_DEFICEFS = n dependency and thus it is impossible to enable user
namespace support in 3.5-rc1.  So remove the now useless depedency.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>