Remove older toolchains am: 2c38fcac06 am: 2c6a96cd8e

Original change: https://android-review.googlesource.com/c/platform/prebuilts/rust/+/1498506

Change-Id: Ifecd2bf450a2dc61592248023eb06122f6fb3e7e
tree: 8b32bc1b0a32a58a2e0fa12ad4fa269a0b24372a
  1. bootstrap/
  2. darwin-x86/
  3. linux-x86/
  4. Android.bp
  5. OWNERS