Remove vectorcall-unwind, it is unstable

https://github.com/rust-lang/rust/blob/c074d8eee765cfd64e6e143d2894c85c7f3ddc1d/compiler/rustc_abi/src/extern_abi/mod.rs#L214-L217

Author: ehuss

src/items/external-blocks.md

@@ -166,7 +166,6 @@ Like `"C"` and `"system"`, most platform-specific ABI strings also have a
 * `"cdecl-unwind"`
 * `"stdcall-unwind"`
 * `"fastcall-unwind"`
-* `"vectorcall-unwind"`
 * `"thiscall-unwind"`
 * `"aapcs-unwind"`
 * `"win64-unwind"`