ALSA: usb-audio: Remove async workaround for Scarlett 2nd gen

Frame size computation has been fixed and the workaround is no longer
needed.

Signed-off-by: Alexander Tsoy <alexander@tsoy.me>
Link: https://lore.kernel.org/r/20200424022449.14972-2-alexander@tsoy.me


Signed-off-by: Takashi Iwai <tiwai@suse.de>