function deleteState(entity, entitySelector) {
  if (this.state[entity][entitySelector]) {
    delete this.state[entity][entitySelector];
  }
}
@description Delete a state.
@param {string} entity - The type of entity we should get the value from.
@param {string} entitySelector - The selector to identify one entity.
@example
stateManager.delete('house', 'main-house');