Remove external/opencensus-java/.gitmodules and submodule directories. am: ac79827f1c am: 8e731716d3 am: 5668bced78 am: d74e0023e1

Original change: https://android-review.googlesource.com/c/platform/external/opencensus-java/+/1830699

Change-Id: If18f27c79b227eab743c3a43a3e4586a701e41a2