Remove input_variables.py

[jkirk5]

Desired capability or behavior.

input_variables.py has not been updated nor is it currently being used, and there are no plans to use it in the future. This file should be removed. Issue #337 is a more modern version of attempting to fix the problem input_variables.py was intended to solve, making it clear what variables are inputs (and which inputs a user needs to provide)

Is your feature request related to a problem? Please describe.

No response

Associated Bug Report

No response