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

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

Change-Id: Ia15951984fba9ce60b45dc1353769bbab07574cd
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>