Remove unneeded core import

This was previously used by the unstable nonzero impls.

    #[cfg(feature = "unstable")]
    use core::num::{NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};

serde/src/lib.rs

@@ -130,9 +130,6 @@
 #[cfg(feature = "alloc")]
 extern crate alloc;
 
-#[cfg(all(feature = "unstable", feature = "std"))]
-extern crate core;
-
 /// A facade around all the types we need from the `std`, `core`, and `alloc`
 /// crates. This avoids elaborate import wrangling having to happen in every
 /// module.