Documentation: remove obsolete voyager.txt file

x86/Voyager support has been removed a year ago.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Inspired-by: Jonathan Corbet <corbet@lwn.net>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>