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

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

Change-Id: I1d2f6b158881bd79368c5b6713428dd4da6e6b1d