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/msroots.rs

@@ -51,11 +51,10 @@ impl MsRoots {
 
     /// Read `msroots` file from YubiKey
     pub fn read(yubikey: &mut YubiKey) -> Result<Option<Self>, Error> {
-        let cb_data = yubikey.obj_size_max();
         let txn = yubikey.begin_transaction()?;
 
         // allocate first page
-        let mut data = Vec::with_capacity(cb_data);
+        let mut data = Vec::with_capacity(CB_OBJ_MAX);
 
         for object_id in YKPIV_OBJ_MSROOTS1..YKPIV_OBJ_MSROOTS5 {
             let buf = txn.fetch_object(object_id)?;
@@ -106,7 +105,6 @@ impl MsRoots {
         let data = &self.0;
         let data_len = data.len();
         let n_objs: usize;
-        let cb_obj_max = yubikey.obj_size_max();
         let txn = yubikey.begin_transaction()?;
 
         if data_len == 0 {
@@ -114,7 +112,7 @@ impl MsRoots {
         }
 
         // Calculate number of objects required to store blob
-        n_objs = (data_len / (cb_obj_max - CB_OBJ_TAG_MAX)) + 1;
+        n_objs = (data_len / (CB_OBJ_MAX - CB_OBJ_TAG_MAX)) + 1;
 
         if n_objs > 5 {
             return Err(Error::SizeError);
@@ -123,8 +121,8 @@ impl MsRoots {
         for i in 0..n_objs {
             offset = 0;
 
-            data_chunk = if cb_obj_max - CB_OBJ_TAG_MAX < data_len - data_offset {
-                cb_obj_max - CB_OBJ_TAG_MAX
+            data_chunk = if CB_OBJ_MAX - CB_OBJ_TAG_MAX < data_len - data_offset {
+                CB_OBJ_MAX - CB_OBJ_TAG_MAX
             } else {
                 data_len - data_offset
             };