tests: Drop test_mpu.py from the PR

The test_mpu.py script is used to verify the equivalence between
existing process group management facilities and the proposed, unified
ParallelState. It is meant to be an temporary helper and will not be
useful after we switch existing implementations to the new interfaces.
Thus remove it from the current PR.

The test is still available at
https://gist.github.com/eternalNight/b76c72216b4be84832b615b76465396f.

Signed-off-by: Junjie Mao <junjie.mao@linux.alibaba.com>

Author: eternalNight