powerpc: Remove -Wattribute-alias pragmas

With SYSCALL_DEFINEx() disabling -Wattribute-alias generically, there's
no need to duplicate that for PowerPC syscalls.

This reverts commit 41552037 ("powerpc: fix build failure by
disabling attribute-alias warning in pci_32") and commit 2479bfc9
("powerpc: Fix build by disabling attribute-alias warning for
SYSCALL_DEFINEx").

Signed-off-by: Paul Burton <paul.burton@mips.com>
Acked-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>