rustc: Remove platform intrinsics crate

This was originally attempted in #57048 but it was realized that we
could fully remove the crate via the `"unadjusted"` ABI on intrinsics.
This means that all intrinsics in stdsimd are implemented directly
against LLVM rather than using the abstraction layer provided here. That
ends up meaning that this crate is no longer used at all.

This crate developed long ago to implement the SIMD intrinsics, but we
didn't end up using it in the long run. In that case let's remove it!

Author: alexcrichton

Cargo.lock

@@ -2564,10 +2564,6 @@ dependencies = [
  "syntax_pos 0.0.0",
 ]
 
-[[package]]
-name = "rustc_platform_intrinsics"
-version = "0.0.0"
-
 [[package]]
 name = "rustc_plugin"
 version = "0.0.0"
@@ -2678,7 +2674,6 @@ dependencies = [
  "rustc 0.0.0",
  "rustc_data_structures 0.0.0",
  "rustc_errors 0.0.0",
- "rustc_platform_intrinsics 0.0.0",
  "rustc_target 0.0.0",
  "smallvec 0.6.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "syntax 0.0.0",