coresight: etm4x: Make etm4_remove_dev() return void

etm4_remove_dev() returned zero unconditionally. Make it return void
instead, which makes it clear in the callers that there is no error to
handle. Simplify etm4_remove_platform_dev() accordingly.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230518201629.260672-1-u.kleine-koenig@pengutronix.de