binutils: remove 2.20.1, 2.21 and 2.23.1

 - The 2.20.x series is old, it no longer makes sense to support it.

 - In the 2.21.x series, we still have 2.21.1, so keeping 2.21 doesn't
   make much sense, so this patch removes it.

 - Similarly for the 2.23.x series, having both 2.23.1 and 2.23.2
   doesn't make much sense, so this patch removes 2.23.1 and keeps
   2.23.2.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>