Revert "ALSA: usb-audio: Add quirk for Focusrite Scarlett

This reverts commit 1762a59d.

This quirk is not needed because support for the Scarlett mixers will be added.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>