Minor: Silence compiler warnings for parquet::file::metadata::reader

[etseidl]

Rationale for this change

When compiling parquet tests with default features, several unused imports are flagged.

% cargo clippy -p parquet --all-targets -- -D warnings
    Checking parquet v53.0.0 (/Users/seidl/src/arrow-rs/parquet)
error: unused import: `std::fs::File`
   --> parquet/src/file/metadata/reader.rs:645:9
    |
645 |     use std::fs::File;
    |         ^^^^^^^^^^^^^
    |
    = note: `-D unused-imports` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_imports)]`

error: unused imports: `Read`, `SeekFrom`, and `Seek`
   --> parquet/src/file/metadata/reader.rs:648:19
    |
648 |     use std::io::{Read, Seek, SeekFrom};
    |                   ^^^^  ^^^^  ^^^^^^^^

error: unused imports: `AtomicUsize` and `Ordering`
   --> parquet/src/file/metadata/reader.rs:649:29
    |
649 |     use std::sync::atomic::{AtomicUsize, Ordering};
    |                             ^^^^^^^^^^^  ^^^^^^^^

What changes are included in this PR?

Only include the above if the "async" feature is enabled.

Are there any user-facing changes?

No

[tustvold]

Perhaps it might be more maintainable to split the async tests into a separate async_tests module or something?

[alamb]

Thanks @etseidl and @tustvold