Drop YubiKey NEO support (closes #18)

YubiKey NEOs are legacy YubiKey devices, most of which contain
unpatchable security vulnerabilities.

They have smaller buffer sizes than YK4 and YK5, which necessitates a
whole bunch of conditional gating and buffer size calculations.

Getting rid of them simplifies this logic and allows us to assume
consistent buffer sizes everywhere.

We never tested on NEOs anyway, and looking at the deleted code it seems
it may have been miscalculating the NEO's buffer size!

If someone *really* wants to support NEOs, it shouldn't be that hard to
restore, but the codebase is definitely cleaner without it.

README.md

@@ -40,11 +40,11 @@ endorsed by Yubico.
 
 ## Supported YubiKeys
 
-- [YubiKey NEO] series (may be dropped in the future, see [#18])
 - [YubiKey 4] series
 - [YubiKey 5] series
 
-NOTE: Nano and USB-C variants of the above are also supported
+NOTE: Nano and USB-C variants of the above are also supported.
+      Pre-YK4 [YubiKey NEO] series is **NOT** supported (see [#18]).
 
 ## Security Warning