docs: device_link.rst: remove duplicated kernel-doc include

The infrastructure.rst file already includes the external
symbols from drivers/base/core.c.

Duplicating 3 functions there causes namespace collisions:

	Documentation/driver-api/device_link.rst: WARNING: Duplicate C declaration, also defined in 'driver-api/infrastructure'.
	Declaration is 'device_link_state'.
	Documentation/driver-api/device_link.rst: WARNING: Duplicate C declaration, also defined in 'driver-api/infrastructure'.
	Declaration is 'device_link_add'.
	Documentation/driver-api/device_link.rst: WARNING: Duplicate C declaration, also defined in 'driver-api/infrastructure'.
	Declaration is 'device_link_del'.
	Documentation/driver-api/device_link.rst: WARNING: Duplicate C declaration, also defined in 'driver-api/infrastructure'.
	Declaration is 'device_link_remove'.

So, drop the reference, adding just a mention to the functions
associated with device_link.

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