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: Ic49a73c4f6aa8e4d16a3413194d7b93dd2f5df3a