xen/arm: Some clean up in time.c

xen/arm: Some clean up in time.c

The file xen/arm/time.c contains some unused code:
    - calibrate_timer: firmware already set correctly CNTFRQ. If it's
    not the case the device tree will containt a property clock-frequency
    in the timer node
    - USE_HYP_TIMER: It's set unconditionally to 1. I didn't see any
    recent model having issue with the hyp timer. I think we can drop
    it safely.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>