Remove mentions of unstable ABIs

Author: gnzlbg

src/items/external-blocks.md

@@ -90,13 +90,6 @@ There are also some platform-specific ABI strings:
 * `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's
   `__vectorcall` and clang's `__attribute__((vectorcall))`
 
-Finally, there are some unstable rustc-specific ABI strings:
-
-* `extern "rust-intrinsic"` -- The ABI of rustc intrinsics, equivalent to `"Rust"`.
-* `extern "rust-call"` -- The ABI of the Fn::call trait functions, equivalent to `"Rust"`.
-* `extern "platform-intrinsic"` -- The ABI of Specific platform intrinsics, equivalent to `"Rust"` -- like, for
-  example, `sqrt` -- equivalent to `"Rust"`. You should never have to deal with it.
-
 ## Variadic functions
 
 Functions within external blocks may be variadic by specifying `...` after one

src/items/functions.md

@@ -175,12 +175,6 @@ with such ABIs causes the process to abort.
 **Non-normative note**: The LLVM backend of the current Rust implementation
 aborts the process by executing an illegal instruction.
 
-**Non-normative note**: There are other ABIs available in unstable Rust that are
-equivalent to the `"Rust"` ABI, e.g., [`"rust-intrinsic"`][rust_intrinsic] or
-[`"platform-intrinsic"`][platform_intrinsic]. For more information about these
-refer to the [ABI section of external block items][external_block_abi] and the
-[Unstable Book].
-
 ## Const functions
 
 Functions qualified with the `const` keyword are const functions. _Const