powerpc: Remove unused 'protect4gb' boot parameter

'protect4gb' boot parameter was introduced to avoid allocating dma
space acrossing 4GB boundary in 2007 (the commit
569975591c5530fdc9c7a3c45122e5e46f075a74).

In 2008, the IOMMU was fixed to use the boundary_mask parameter per
device properly. So 'protect4gb' workaround was removed (the
383af9525bb27f927511874f6306247ec13f1c28). But somehow I messed the
'protect4gb' boot parameter that was used to enable the
workaround.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>