`StateMap` remove the "setting an out of bounds index expands the entire list" feature

[pinkwah]

state_map = StateMap()
len(state_map)  #=> 0
state_map[10] = RealizationStateEnum.STATE_HAS_DATA
len(state_map)  #=> 11

[oyvindeide]

Blocked by: https://github.com/equinor/ert/pull/3201

[dafeda]

#3201 is closed so this can be unblocked I think.