powerpc: Book E: Remove unused CPU_FTR_L2CSR bit

The CPU_FTR_L2CSR bit is never tested anywhere, so let's reclaim the
bit.

The last usage was removed in 86d63363 ("powerpc/e500mc: Remove
dead L2 flushing code in idle_e500.S") (Jun 2015).

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>