Remove sys_nice capability from domains.

Remove sys_nice capability from domains; this does not appear to be necessary
and should not be possible in particular for app domains.  If we encounter
specific instances where it should be granted, we can add it back on a
per-domain basis.  Allow it explicitly for the system_server.  Unconfined
domains get it via unconfined_domain() and the rules in unconfined.te.

Change-Id: I9669db80a04a90a22241b2fbc5236a28dcde8c6e
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>