powerpc: Remove leftover cputime_to_nsecs call causing build error

This type conversion is a leftover that got ignored during the kcpustat
conversion to nanosecs, resulting in build breakage with config having
CONFIG_NO_HZ_FULL=y.

	arch/powerpc/kernel/time.c: In function 'running_clock':
	arch/powerpc/kernel/time.c:712:2: error: implicit declaration of function 'cputime_to_nsecs' [-Werror=implicit-function-declaration]
	  return local_clock() - cputime_to_nsecs(kcpustat_this_cpu->cpustat[CPUTIME_STEAL]);

All we need is to remove it.

Fixes: e7f340ca

 ("powerpc, sched/cputime: Remove unused cputime definitions")
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>