Remove CLOEXEC workarounds for Linux 2.6.18.

Rust itself has updated its minimum supported versison to
2.6.32 [here](rust-lang/rust#74606).

Fixes #85.

cap-primitives/src/rsix/fs/open_unchecked.rs

@@ -7,7 +7,7 @@ use rsix::io;
 use std::fs;
 use std::path::Path;
 #[cfg(any(target_os = "android", target_os = "linux"))]
-use {crate::fs::ensure_cloexec, io_lifetimes::AsFd};
+use io_lifetimes::AsFd;
 
 /// *Unsandboxed* function similar to `open`, but which does not perform
 /// sandboxing.
@@ -23,9 +23,6 @@ pub(crate) fn open_unchecked(
 
     let err = match openat(start, path, oflags, mode) {
         Ok(file) => {
-            #[cfg(any(target_os = "android", target_os = "linux"))]
-            ensure_cloexec(file.as_fd()).map_err(OpenUncheckedError::Other)?;
-
             return Ok(fs::File::from_fd(file.into()));
         }
         Err(err) => err,

cap-primitives/src/rsix/linux/fs/mod.rs

@@ -1,5 +1,4 @@
 mod canonicalize_impl;
-mod ensure_cloexec;
 mod file_metadata;
 mod file_path;
 mod open_entry_impl;
@@ -11,7 +10,6 @@ mod stat_impl;
 
 pub(crate) use crate::fs::via_parent::set_times_nofollow as set_times_nofollow_impl;
 pub(crate) use canonicalize_impl::canonicalize_impl;
-pub(crate) use ensure_cloexec::ensure_cloexec;
 pub(crate) use file_path::file_path;
 pub(crate) use open_entry_impl::open_entry_impl;
 pub(crate) use open_impl::{open_beneath, open_impl};

cap-primitives/src/rsix/linux/fs/open_impl.rs

@@ -74,9 +74,6 @@ pub(crate) fn open_beneath(
             ) {
                 Ok(file) => {
                     let file = fs::File::from_into_fd(file);
-                    // Note that we don't bother with `ensure_cloexec` here
-                    // because Linux has supported `O_CLOEXEC` since 2.6.18,
-                    // and `openat2` was introduced in 5.6.
 
                     #[cfg(racy_asserts)]
                     check_open(start, path, options, &file);