Re-delete zombie 'drivers/usb/serial/airprime.c' file

Stephen Rothwell points out that this file got deleted (on purpose) by
commit 640c1bc ("USB: delete airprime
driver"), but then almost immediately incorrectly resurrected by commit
95da310 ("usb_serial: API all change").

Delete it again.  If it comes back, we'll need to drive a stake through
its heart.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Greg KH <greg@kroah.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>