Revert^4 "add rust_toolchain_rustc_prebuilt module type" am: f2b16069 am:...

Revert^4 "add rust_toolchain_rustc_prebuilt module type" am: f2b16069 am: 942a6409 am: b875e715

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/2761805



Change-Id: I62f40d34d5e114846d51be141df1a52e93b8b218
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>