powerpc/64e: remove mmu_linear_psize

mmu_linear_psize is only set at boot once on 64e, is not necessarily
the correct size of the linear map pages, and is never used anywhere.
Remove it.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Retain the extern, so we can use IS_ENABLED() for related code]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211201144153.2456614-14-npiggin@gmail.com