Remove submodules

The Zephyr/Pigweed developers want to use Zephyr's "west" tool which
does many things but for our purposes is like the Android Repo Tool.
Pigweed has no intention of supporting west but can mimic it with an
Android Repo Tool project with the same directory structure. Removing
the submodules because, for Pigweed, these will not be used.

Change-Id: I1171ffc6d8a517449c78fb115fac4c8485308799
Bug: b/267213895
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/139270
Reviewed-by: Erik Gilling <konkers@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
3 files changed