stty: remove redundant cfg

[cakebaker]

I noticed the following snippet in our code:

#[cfg(any(
    target_os = "android",
    target_os = "haiku",
    target_os = "ios",
    target_os = "linux",
    target_os = "macos"
))]
#[cfg(any(
    target_os = "android",
    target_os = "haiku",
    target_os = "ios",
    target_os = "linux",
    target_os = "macos"
))]
Flag::new_grouped("tab1", O::TAB1, O::TABDLY),

This PR removes the redundant cfg.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)