powerpc/mm: remove unused function prototype

Commit f384796c

 ("powerpc/mm: Add support for handling > 512TB address
in SLB miss") removed function slb_miss_bad_addr(struct pt_regs *regs), but
kept its declaration in the prototype file. This patch simply removes the
function definition.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>