ALSA: usb-audio: Drop keep_interface flag again

This behavior turned out to be invalid from the USB spec POV and
shouldn't be applied.  As it's an optional flag that is set only via
an card control element that must be hardly used, let's drop it
again.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-16-tiwai@suse.de


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