powerpc/kernel: Remove the unused extern dscr_default

The process context switch code no longer uses dscr_default variable
from the sysfs.c file. The variable became unused when we started
storing the CPU specific DSCR value in the PACA structure instead.
This patch just removes this extern declaration. It was originally
added by the following commit.
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -1112,7 +1112,6 @@ static void setup_ksp_vsid(struct task_struct *p, unsigned long sp)
 /*
  * Copy a thread..
  */
-extern unsigned long dscr_default; /* defined in arch/powerpc/kernel/sysfs.c */
 
 /*
  * Copy architecture-specific thread state