chore: remove outdated example (#671)

This example is outdated and should have been removed when `tracing-fmt`
was merged into `tracing-subscriber`. There is another example in the
`examples` crate, [`fmt-custom-fields`][1], which demonstrates the same
usage, but uses the current APIs. Therefore, we can just delete the old
example.

[1]: https://github.com/tokio-rs/tracing/blob/master/examples/examples/fmt-custom-field.rs

Fixes: #444

Signed-off-by: Eliza Weisman <eliza@buoyant.io>