Make Exit(), GetLaunchDir() and SConscriptChdir() static

[abitrolly]

Continuation of #4224 which is a consequence of #4223

• Do not initialize DefaultEnvironment when calling EnsureSConsVersion(),
  EnsurePythonVersion(), Exit(), GetLaunchDir() and SConscriptChdir().
• Remove unused private method SConsEnvironment._exceeds_version().

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation

[bdbaddog]

Doing this would require a deprecation cycle.

[abitrolly]

For the private method that is not supposed to be exposed?

[bdbaddog]

For the private method that is not supposed to be exposed?

Removing those methods from Environment()..

[abitrolly]

This PR doesn't remove them. Should it start the deprecation cycle now that the docs moved methods into global scope?

[bdbaddog]

You are correct. My mistake.