Remove external/opencensus-java/.gitmodules and submodule directories. am: ac79827f1c am: 8e731716d3 am: 5668bced78 am: d74e0023e1 am: 25e068999f

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

Change-Id: I3ecff914fad2d0a38fcd73382e6bf4a87a0a67b0