Modify Objective::_erase_function_variables(...) to also delete varia…

[Hesam-Vayu]

…ble name from _all_variables.

Not deleting a variable name in _all_variables that is no longer used causes crashes when you try to add a CostFunction that includes the variable name again.

Motivation and Context

If you delete enough CostFunctions where the underlying variables are deleted and then add another CostFunction that uses the same variable, the following check passes, causing this statement to crash due to this statement not being executed.

How Has This Been Tested

Ran tests and optimizations in own code.

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

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

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[mhmukadam]

Thanks for catching this and the fix. Might be good to update the unit test so this delete behavior also gets checked.

[Hesam-Vayu]

Done, I added the test.

[Hesam-Vayu]

My pleasure :)