maintainers: remove gmarull as a PM subsystem collaborator

I disagree on the way the subsystem is maintained. For more context
refer to https://github.com/zephyrproject-rtos/zephyr/pull/61726 or
https://discord.com/channels/720317445772017664/997527108844798012/


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Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>