powerpc/e500mc: Remove dead L2 flushing code in idle_e500.S

This code can never be executed as it is only built when
CONFIG_PPC_E500MC is unset, but the only CPUs that have CPU_FTR_L2CSR
require CONFIG_PPC_E500MC and do not have the MSR/HID0-based nap
mechanism that this file uses.

Signed-off-by: Scott Wood <scottwood@freescale.com>