Revert "modules: nanopb: Move pip dependencies to optional module"

This reverts commit 26614832.
A bug in upstream nanopb, which this commit was pulling as part
of the submanifest update, seems to be causing test failures
on big-endian platforms.
Upstream bug: https://github.com/nanopb/nanopb/issues/1039



Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>