Remove external/libcxx in favor of prebuilt am: 3f93ecd0e1 am: 350a7a935e

Original change: https://android-review.googlesource.com/c/platform/external/libcxx/+/2675624

Change-Id: Iead0e40e831369edfe40459c61fc052366b5497e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>