powerpc: Remove stale declarations in mmu_decl.h

rtas_size and rtas_data are not used anymore since at least
commit 7c8c6b97 ("powerpc: Merge lmb.c and make MM initialization
use it.")

Remove them.

Since commit 4b74a35f ("powerpc/32s: Make Hash var static")
the forward declaration of struct hash_pte is unneeded.

Remove it.

__initial_memory_limit_addr was removed by commit e63075a3 ("memblock:
Introduce default allocation limit and use it to replace explicit ones")

Remove the declaration.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a821e8397dd56b8177ecc04966d3b3a7c4bda6d4.1660919016.git.christophe.leroy@csgroup.eu