Revert "bp2build allowlist //external/libcxx/..." am: f4afb53d am:...

Revert "bp2build allowlist //external/libcxx/..." am: f4afb53d am: 0b81acd5 am: b0f859d2 am: 58b17bc8

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1814738

Change-Id: Ia657261bd01c73649eecd248211206acaa9d284d