clocksource/drivers/efm32: Drop unused timer code

Support for this machine was just removed, so drop the now unused timer
code, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210115155130.185010-4-u.kleine-koenig@pengutronix.de