powerpc: Remove unused function trace_syscall()

This function has been unused since commit 14cf11af ("powerpc: Merge enough
to start building in arch/powerpc."), so remove it.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>