If ARCH_SPARSEMEM_ENABLE=y and ARCH_{FLATMEM,DISCONTIGMEM}_ENABLE=n,
then the logic in mm/Kconfig already makes CONFIG_SPARSEMEM the only
choice. This is true for all of the existing ARM users of
ARCH_SPARSEMEM_ENABLE.
Forcing ARCH_SPARSEMEM_DEFAULT=y if ARCH_SPARSEMEM_ENABLE=y prevents
us from ever defaulting to FLATMEM, so we should remove this setting.