ARM: plat-versatile: Remove unused clock.c file

This file isn't compiled anymore because PLAT_VERSATILE_CLOCK is
never selected. Remove the file and the config.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>