Revert "bp2build: build //system/core/libprocessgroup:libprocess..."

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: Idcacf7b8e48e1cd156717d6b7a6d1f9534207d49