Remove linuxkernel targets

These are not used by the actual Rust-for-Linux project, so they're mostly just confusing.

Author: alex

unwind/src/lib.rs

@@ -15,7 +15,6 @@ cfg_if::cfg_if! {
         target_os = "espidf",
     ))] {
         // These "unix" family members do not have unwinder.
-        // Note this also matches x86_64-unknown-none-linuxkernel.
     } else if #[cfg(any(
         unix,
         windows,