docs: kernel-api.rst: drop kernel/irq/manage.c kernel-doc tag

docs: kernel-api.rst: drop kernel/irq/manage.c kernel-doc tag

This is already included at genericirq.rst. Adding it twice
causes C namespace duplication:

	.../Documentation/core-api/kernel-api:237: ../kernel/irq/manage.c:100: WARNING: Duplicate C declaration, also defined in 'genericirq'.
	Declaration is 'synchronize_hardirq'.
	.../Documentation/core-api/kernel-api:237: ../kernel/irq/manage.c:128: WARNING: Duplicate C declaration, also defined in 'genericirq'.
	Declaration is 'synchronize_irq'.
	.../Documentation/core-api/kernel-api:237: ../kernel/irq/manage.c:443: WARNING: Duplicate C declaration, also defined in 'genericirq'.
	Declaration is 'irq_set_affinity_notifier'.
	...

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>