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: I53c69a24eb28fd72a9ab4408c4ec32781c2d6f6b