Revert "allow system_server to set kernel scheduling priority"

Periodically, SELinux denials of the form:

  type=1400 audit(0.0:8574): avc: denied { setsched } for comm="system_server" scontext=u:r:system_server:s0 tcontext=u:r:kernel:s0 tclass=process permissive=0

are being generated. These denials come from system_server and other
processes. There's no reason why system_server should be calling
sched_setscheduler() on a kernel thread.

Current belief is that these SELinux denials are a bug in the kernel,
and are being inappropriately triggered.

Revert 2d1650f4. The original reason
for accepting this change was to see if it would fix bug 18085992.
Unfortunately, even after the commit, the bug was still present.
The change had no impact on the bug.

Don't inappropriately grant system_server the ability to minipulate
the scheduling priority of kernel threads.

This reverts commit 2d1650f4.

Change-Id: I59bdf26ad247a02b741af2fa58a18e7e83ef44d8