arc: drop definitions of pgd_index() and pgd_offset{, _k}() entirely

They were in <asm/pgtables.h> and have been removed from there in
974b9b2c ("mm: consolidate pte_index() and pte_offset_*() definitions")
in favor of the generic version. But that missed that the same definitons
also existed in <asm/pgtable-levels.h>, where they were (inadvertently?)
introduced in fe6cb7b0 ("ARC: mm: disintegrate pgtable.h into levels
and flags").

Fixes: 974b9b2c ("mm: consolidate pte_index() and pte_offset_*() definitions")
Fixes: fe6cb7b0 ("ARC: mm: disintegrate pgtable.h into levels and flags")
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>