Clean up unnecessary cfg_attr attributes

src/unix/bsd/freebsdlike/mod.rs

src/unix/bsd/mod.rs

src/unix/haiku/mod.rs

src/unix/linux_like/android/mod.rs

src/unix/linux_like/linux/gnu/mod.rs

src/unix/linux_like/linux/mod.rs

src/unix/newlib/mod.rs

src/unix/uclibc/mod.rs