ENV_OBJECT_DIRECT_IO_ENABLE, DEFAULT_OBJECT_DIRECT_IO_ENABLE,
ENV_OBJECT_DIRECT_IO_THRESHOLD and DEFAULT_OBJECT_DIRECT_IO_THRESHOLD
were never read anywhere in the workspace. The real O_DIRECT read path
landed in PR #4365 uses RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE /
RUSTFS_OBJECT_DIRECT_IO_READ_THRESHOLD (crates/ecstore/src/disk/local.rs).
Keeping the near-identically named dead knobs invites operators to set
the wrong variable and believe O_DIRECT is enabled.
Co-authored-by: heihutu <[email protected]>
