powerpc/mm: Remove unused register usage in SW TLB miss handling

Long ago we had some code that actually used the CTR in the SW TLB
miss handlers (603/e300).  Since we don't use it no reason to waste
cycles saving it off and restoring it (we actually didn't restore it
in the fast path case).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>