Documentation: Mark the 'efivars' sysfs interface as removed

The 'efivars' sysfs interface was removed in commit 0f5b2c69

 ("efi:
vars: Remove deprecated 'efivars' sysfs interface"), but the ABI
documentation was not updated properly.

Strip down the documentation file for /sys/firmware/efi/vars to a very
basic description of what the interface was about, add a section about
the rough removal timeline, and inform the reader about the intended
replacement.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>