ARM: local timers: Remove CONFIG_LOCAL_TIMERS support

Local timer support in arch/arm is now completely unused, as all
in-tree platforms are using standalone timers.

It allows us to simplify the code, only preserving the broadcast
timer. The broadcast timer is always installed, and we leave it up
to the kernel to pick the more suitable timer (usually the one with
the highest priority).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>