Remove all submodules.

Submodules are not supported by the current Android platfrom development tool chain, so only make git do extra work. Even worse, misconfigured submodules that break git-based workflows can be submitted without triggering a presubmit failure.

Test: TreeHugger
Issue: 109878533
Change-Id: I95d7c60296cb37184f94457f4f8abac9910cc88a