clocksource/drivers/u300: Remove the u300 driver

The ST-Ericsson U300 platform is getting removed, so this driver is no
longer needed.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210120131559.1971359-2-arnd@kernel.org