sched: Make sched_set{scheduler, attr}() static

Now all external users are using alternative exported wrappers, hide the
definition of these functions.

The rationale behind that is that it is hard to reason in the kernel
about an optimal correct settings for a task given the wide range of
systems the kernel can run on.

In-kernel users are legitimate still, but by providing correct wrappers
we ensure they're sane and consistent. Hence producing no surprise
results or extra level of management to userspace.

The two main concerns are:

	1. Being able to set pseudo-random RT priorities which make no
	   sense outside of the system and workload it was tried on.

	2. Being able to set pseudo-random uclamp min/max values which
	   also don't make a lot of sense outside of the system and
	   workload it was tried on.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>