removing unused attribute, adding initialization of _fog_of_war_mask

[naokiyokoyama]

Motivation and Context

The attribute on line 698 (self._grid_delta) is not used at all anywhere in the repo, so it can be deleted.
On the other hand, self._fog_of_war_mask IS used a lot but is NOT in the __init__, which confuses IDEs like PyCharm when writing code for inheriting this class and trying to access/edit self._fog_of_war_mask. Thus it is added here.

How Has This Been Tested

Non-breaking changes

Types of changes

• Docs change / refactoring / dependency upgrade

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.