Unset a variable, cache variable, or environment variable.
Removes the specified variable causing it to become undefined. If
CACHE is present then the variable is removed from the cache instead
of the current scope.
PARENT_SCOPE is present then the variable is removed from the scope
above the current scope. See the same option in the :command:
for further details.
<variable> can be an environment variable such as:
in which case the variable will be removed from the current environment.