sched: Remove USER_PRIO, TASK_USER_PRIO and MAX_USER_PRIO

The only remaining use of MAX_USER_PRIO (and USER_PRIO) is the
SCALE_PRIO() definition in the PowerPC Cell architecture's Synergistic
Processor Unit (SPU) scheduler. TASK_USER_PRIO isn't used anymore.

SCALE_PRIO() was copied from the task scheduler in commit fe443ef2
("[POWERPC] spusched: Dynamic timeslicing for SCHED_OTHER") in v2.6.23.

Then it was removed from the task scheduler in commit a4ec24b4
("sched: tidy up SCHED_RR") in v2.6.24.

NICE_WIDTH was introduced much later in commit 3ee237dd
("sched/prio: Add 3 macros of MAX_NICE, MIN_NICE and NICE_WIDTH in
prio.h").

With:

    MAX_USER_PRIO = USER_PRIO(MAX_PRIO) = MAX_PRIO - MAX_RT_PRIO

         MAX_PRIO = MAX_RT_PRIO + NICE_WIDTH

    MAX_USER_PRIO = MAX_RT_PRIO + NICE_WIDTH - MAX_RT_PRIO

    MAX_USER_PRIO = NICE_WIDTH

MAX_USER_PRIO can be replaced by NICE_WIDTH to be able to remove all the
{*_}USER_PRIO defines.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>