avro

Remove unnecessary cast (clippy warning) (#172)

With the upgrade to Rust 1.49, the clippy lint `unnecessary_cast` was
[enhanced to work with integer and float literals][0]. There is one
instance of this in the codebase that is breaking the build.

This change removes the cast and fixes the clippy warning.

[0]: https://github.com/rust-lang/rust-clippy/blob/00586dfdcd10c37cb8b132c72ed0558304955042/CHANGELOG.md#enhancements-1