powerpc/mm: Remove DEBUG_VM_PGTABLE support on powerpc

The test is broken w.r.t page table update rules and results in kernel
crash as below. Disable the support until we get the tests updated.

[   21.083519] kernel BUG at arch/powerpc/mm/pgtable.c:304!
cpu 0x0: Vector: 700 (Program Check) at [c000000c6d1e76c0]
    pc: c00000000009a5ec: assert_pte_locked+0x14c/0x380
    lr: c0000000005eeeec: pte_update+0x11c/0x190
    sp: c000000c6d1e7950
   msr: 8000000002029033
  current = 0xc000000c6d172c80
  paca    = 0xc000000003ba0000   irqmask: 0x03   irq_happened: 0x01
    pid   = 1, comm = swapper/0
kernel BUG at arch/powerpc/mm/pgtable.c:304!
[link register   ] c0000000005eeeec pte_update+0x11c/0x190
[c000000c6d1e7950] 0000000000000001 (unreliable)
[c000000c6d1e79b0] c0000000005eee14 pte_update+0x44/0x190
[c000000c6d1e7a10] c000000001a2ca9c pte_advanced_tests+0x160/0x3d8
[c000000c6d1e7ab0] c000000001a2d4fc debug_vm_pgtable+0x7e8/0x1338
[c000000c6d1e7ba0] c0000000000116ec do_one_initcall+0xac/0x5f0
[c000000c6d1e...