Delete nonprintable characters from aws-smithy-checksums test (#3778)

## Motivation and Context

The nonprintable characters in this source file make tools misidentify
it as a binary file, rather than source code.

## Description

**Before:**

```console
$ file rust-runtime/aws-smithy-checksums/src/http.rs
rust-runtime/aws-smithy-checksums/src/http.rs: data
```

**After:**

```console
$ file rust-runtime/aws-smithy-checksums/src/http.rs
rust-runtime/aws-smithy-checksums/src/http.rs: ASCII text
```

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