clocksource: Remove obsolete CLOCKSOURCE_OF_DECLARE

The macro CLOCKSOURCE_OF_DECLARE was renamed more TIMER_OF_DECLARE, and we
kept an alias CLOCKSOURCE_OF_DECLARE in order to smooth the transition for
drivers.

This change was done 1.5 year ago, we can reasonably remove this backward
compatible macro as it is no longer used anywhere.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>