Remove usages of YKPIV_OBJ_MAX_SIZE

...replacing them with `CB_BUF_MAX`.

Both constants are 3072, however `CB_BUF_MAX` is what the original code
was using.

See discussion here:

https://github.com/tarcieri/yubikey-piv.rs/pull/17#discussion_r350166104

src/consts.rs

@@ -229,8 +229,6 @@ pub const YKPIV_OBJ_MSROOTS3: u32 = 0x005f_ff13;
 pub const YKPIV_OBJ_MSROOTS4: u32 = 0x005f_ff14;
 pub const YKPIV_OBJ_MSROOTS5: u32 = 0x005f_ff15;
 
-pub const YKPIV_OBJ_MAX_SIZE: usize = 3072;
-
 pub const YKPIV_PINPOLICY_TAG: u8 = 0xaa;
 pub const YKPIV_PINPOLICY_DEFAULT: u8 = 0;
 pub const YKPIV_PINPOLICY_NEVER: u8 = 1;