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.

src/certificate.rs

@@ -145,17 +145,15 @@ impl Certificate {
     /// Write this certificate into the YubiKey in the given slot
     #[cfg(feature = "untested")]
     pub fn write(&self, yubikey: &mut YubiKey, slot: SlotId, certinfo: u8) -> Result<(), Error> {
-        let max_size = yubikey.obj_size_max();
         let txn = yubikey.begin_transaction()?;
-        write_certificate(&txn, slot, Some(&self.data), certinfo, max_size)
+        write_certificate(&txn, slot, Some(&self.data), certinfo)
     }
 
     /// Delete a certificate located at the given slot of the given YubiKey
     #[cfg(feature = "untested")]
     pub fn delete(yubikey: &mut YubiKey, slot: SlotId) -> Result<(), Error> {
-        let max_size = yubikey.obj_size_max();
         let txn = yubikey.begin_transaction()?;
-        write_certificate(&txn, slot, None, 0, max_size)
+        write_certificate(&txn, slot, None, 0)
     }
 
     /// Initialize a local certificate struct from the given bytebuffer
@@ -244,7 +242,6 @@ pub(crate) fn write_certificate(
     slot: SlotId,
     data: Option<&[u8]>,
     certinfo: u8,
-    max_size: usize,
 ) -> Result<(), Error> {
     let mut buf = [0u8; CB_OBJ_MAX];
     let mut offset = 0;
@@ -261,7 +258,7 @@ pub(crate) fn write_certificate(
     req_len += set_length(&mut buf, data.len());
     req_len += data.len();
 
-    if req_len < data.len() || req_len > max_size {
+    if req_len < data.len() || req_len > CB_OBJ_MAX {
         return Err(Error::SizeError);
     }