powerpc/configs/64s: Use EXT4 to mount EXT2 filesystems

The ext4 code will mount ext2 filesystems, no need to build in both.

Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230420051609.1324201-1-mpe@ellerman.id.au