Revert "bp2build allowlist //external/libcxx/..."

Revert submission 1814446

Reason for revert: causes failure of mixed_droid
Reverted Changes:
If5285f161:bp2build allowlist //external/libcxx/...
I4e7787374:bp2build: build //system/core/libprocessgroup:libp...
Ibec6d5fbe:bp2build: allowlist //external/brotli/... and buil...

Change-Id: I37327057a3df44cbb6e57d7f54120b3448da6057