clocksource: Remove metag generic timer driver

Now that arch/metag/ has been removed, remove the metag generic
per-thread timer driver. It is of no value without the architecture
code.

Signed-off-by: James Hogan <jhogan@kernel.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-metag@vger.kernel.org