Those are already defined at kernel-api.rst, as part of the
synchronization primitives chapter.

This solves several Sphinx 3 warnings, like:

	.../Documentation/driver-api/basics.rst: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'rcu_idle_enter'.
	.../Documentation/driver-api/basics.rst: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'rcu_idle_exit'.
	.../Documentation/driver-api/basics.rst: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'rcu_is_watching'.
	.../Documentation/driver-api/basics.rst: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'call_rcu'.
	.../Documentation/driver-api/basics.rst: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'synchronize_rcu'.
	...
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>