powerpc: Drop old comment about CONFIG_POWER

There's a comment in time.h referring to CONFIG_POWER, which doesn't
exist. That confuses scripts/checkkconfigsymbols.py.

Presumably the comment was referring to a CONFIG_POWER vs CONFIG_PPC,
in which case for CONFIG_POWER we would #define __USE_RTC to 1. But
instead we have CONFIG_PPC_BOOK3S_601, and these days we have
IS_ENABLED().

So the comment is no longer relevant, drop it.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131728.1643966-9-mpe@ellerman.id.au

arch/powerpc/include/asm/time.h

@@ -39,7 +39,6 @@ struct div_result {
 };
 
 /* Accessor functions for the timebase (RTC on 601) registers. */
-/* If one day CONFIG_POWER is added just define __USE_RTC as 1 */
 #define __USE_RTC()	(IS_ENABLED(CONFIG_PPC_BOOK3S_601))
 
 #ifdef CONFIG_PPC64