In bug-28430 HA_PRIMARY_KEY_REQUIRED_FOR_POSITION
was disabled in the partitioning engine in the first patch,
That bug was later fixed a second time, but that flag
was not removed.

No need to disable this flag, as it leads to bad
choise in row replication.

sql/ha_partition.h:
  Not disabling HA_PRIMARY_KEY_REQUIRED_FOR_POSITION flag.
  Updated comment (has nothing to do with hidden key.
sql/handler.h:
  Updated comments to about HA_PRIMARY_KEY_REQUIRED_FOR_POSITION.