MN10300: Remove stale code

mn10300 implements clocksource and clockevents and selects them
unconditionally in Kconfig. Remove the stale code which seems to be a
leftover of the conversion. Cleanup the configuration switches as well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
Cc: Mark Salter <msalter@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>