clocksource/drivers/sp804: Remove unused sp804_timer_disable() and timer-sp804.h

Since commit 7484c727 ("ARM: realview: delete the RealView board
files") and commit 16956fed ("ARM: versatile: switch to DT only
booting and remove legacy code"), there's no one to use the functions
defined or declared in include/clocksource/timer-sp804.h. Delete it.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200918132237.3552-3-thunder.leizhen@huawei.com