mhp_flag MHP_OFFLINE_INACCESSIBLE was used to mark memory as not
accessible until memory hotplug online phase begins.
Earlier, standby memory blocks were added upfront during boottime and
MHP_OFFLINE_INACCESSIBLE flag avoided page_init_poison() on memmap
during mhp addition phase.
However with dynamic runtime configuration of memory, standby memory can
be brought to accessible state before performing add_memory(). Hence,
remove MHP_OFFLINE_INACCESSIBLE.