MAINTAINERS: Remove Mathieu Poirier as coresight maintainer

Mathieu Poirier is no longer involved in maintainig the CoreSight self-hosted
tracing subsystem.

Mathieu, Thank you very much creating and maintaing the subsystem all these
years !

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20230404111117.569795-1-suzuki.poulose@arm.com