powerpc/smp: remove the incorrect decrementer initial codes for AP

We already defined start_cpu_decrementer() to invoke decrementer for AP as
the following path:

start_secondary() -> secondary_cpu_time_init() -> start_cpu_decrementer()

So remove these incorrect codes introduced from commit:
e7f75ad0 powerpc/47x: Base ppc476 support

And actually we really should not enable decrementer before calling set_dec().

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>