Anything involving a PSDrive, including ENV:, will only affect the current process. But - and I’ll explain my process here, in case that helps you - I Googled “.NET Environment Class.” That got me to https://msdn.microsoft.com/en-us/library/system.environment(v=vs.110).aspx, which is the class you were using to set a variable.
I clicked on Methods, which got me to https://msdn.microsoft.com/en-us/library/system.environment_methods(v=vs.110).aspx.
I didn’t see a “RemoveEnvironmentVariable” method, which made me sad. So I looked at the docs for SetEnvironmentVariable. Specifically, https://msdn.microsoft.com/en-us/library/96xafkes(v=vs.110).aspx.
That told me that the method could also DELETE a variable. Winner! You just pass $null as the value, and if the variable exists, it gets deleted.
Hope that helps.