[ios] Remove obsolete toolchains

The following toolchains were forcing the deployment target to
iOS 16.0 and were used by //ios/intents sample which has been
removed with https://crrev.com/c/7041742 so they are now obsolete.

  //build/toolchain/ios:ios_clang_arm64_16_0
  //build/toolchain/ios:ios_clang_arm64e_16_0
  //build/toolchain/ios:ios_clang_x64_16_0

Change-Id: Iea3ed1d4c0f58b78007adf0f6422906ca2df7a46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7044607
Reviewed-by: Mark Mentovai <mark@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530131}