powerpc/rtas-rtc: remove unused constant

RTAS_CLOCK_BUSY is unused, remove it.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210503175811.1528208-1-nathanl@linux.ibm.com