mpi: Drop dual mode and tweak overlap2/full modes for GPU backends #454

Job	Run time
pytest-mpi (pytest-mpi-docker-gcc)	42s
pytest-mpi (3.10)	1m 56s
pytest-mpi (3.11)	1m 58s
pytest-mpi (pytest-mpi-docker-icx)	1m 6s
pytest-mpi (pytest-mpi-docker-gcc-arm)	1m 6s
pytest-mpi (pytest-mpi-docker-gcc-arm)	2m 48s
pytest-mpi (pytest-mpi-docker-gcc)	1m 53s
pytest-mpi (3.11)	46m 29s
pytest-mpi (pytest-mpi-docker-icx)	2m 48s
pytest-mpi (3.10)	44m 0s
pytest-mpi (pytest-mpi-docker-icx)	33m 20s
pytest-mpi (pytest-mpi-docker-gcc-arm)	20m 59s
pytest-mpi (pytest-mpi-docker-gcc)	29m 55s
pytest-mpi (3.10)	44m 0s
pytest-mpi (3.11)	46m 29s
	4h 39m 29s

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