powerpc/mmu: Remove leftover CONFIG_E200

Commit 39c8bf2b ("powerpc: Retire e200 core (mpc555x processor)")
removed CONFIG_E200.
Commit f9158d58 ("powerpc/mm: Add mask of always present MMU
features") was merged in the same cycle and added a new use of
CONFIG_E200.

Remove that use.

Fixes: f9158d58 ("powerpc/mm: Add mask of always present MMU features")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/594fa3cc0df11e21644fd6a584851ae4f164b2bf.1620367249.git.christophe.leroy@csgroup.eu