Remove EnvironmentVariable helper class

[tlento]

This class is doing a whole lot to accomplish very little. In
updating the sql engine url environment variable access I noticed
the old EnvironmentVariable helper was hardly being used at all
anymore. Since most of its usage has disappeared into the sands
of time, we remove the last remaining calls - and the class
itself - here.

[tlento]

Current dependencies on/for this PR:

• main
  • PR Update Pandas dependency to 1.5.x #719
    • PR Loosen version pins for MetricFlow dependencies #720
      • PR Make test execution more consistent and convenient #721
        • PR Remove EnvironmentVariable helper class #722 👈
          • PR Render date literals as strings in time range filters #724

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[plypaul]

This class was previously used in the server implementation, so the MFS team might need to move it.

[tlento]

This class was previously used in the server implementation, so the MFS team might need to move it.

I checked into that, they're not using it at all.