binutils: remove dead code

We no longer have any way of using version 2.23, and all the other
versions that we support are available as .tar.bz2. Therefore we can
remove the special case related to the 2.23 version only being
available as a .tar.gz archive.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>