ppc64: remove duplicated mb() and comment from __cpu_up

diff -rU 5 linux-2.6.6-rc2-mm1/arch/ppc64/kernel/smp.c linux-2.6.6-rc2-mm1.new/arch/ppc64/kernel/smp.c
@@ -893,14 +893,10 @@
 		memset(tmp, 0, PAGE_SIZE); 
 		paca[cpu].xStab_data.virt = (unsigned long)tmp;
 		paca[cpu].xStab_data.real = virt_to_abs(tmp);
 	}
 
-	/* The information for processor bringup must be written out
-	 * to main store before we release the processor. */
-	mb();
-
 	/* The information for processor bringup must
 	 * be written out to main store before we release
 	 * the processor.
 	 */
 	mb();