kani: Remove redundant import

`cargo` is reporting a redundant import warning:

 warning: the item `TryInto` is imported redundantly

Remove the import statement from the `verification` module.

units/src/amount.rs

@@ -1885,7 +1885,6 @@ pub mod serde {
 #[cfg(kani)]
 mod verification {
     use std::cmp;
-    use std::convert::TryInto;
 
     use super::*;