staging: line6: drop CONFIG_LINE6_USB_DUMP_PCM

The CONFIG_LINE6_USB_DUMP_PCM config option prints a hexdump of PCM
audio data as URBs are sent and received.  The usbmon feature should be
used instead of manually dumping PCM URBs.  There are a few advantages
to using usbmon:

 * Can be turned on/off at runtime
 * Provides full USB-level traffic
 * tcpdump and wireshark support for powerful analysis
 * No driver-specific code is required

This is the last user of line6_write_hexdump() so we drop it too.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>