powerpc/32: Remove wii_memory_fixups()

wii_memory_fixups() is not called anymore, remove it.

Also remove left-overs in mmu_decl.h which were forgotten by
commit 160985f3

 ("powerpc/wii: remove wii_mmu_mapin_mem2()")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5f2091f86528b59ef92ef1daed5d3dd8c0d7bebd.1661938317.git.christophe.leroy@csgroup.eu