Minor clean-ups and simplifications in ExecutionEnvironment

[HannesWell]

What it does

Clean-up the code in the org.eclipse.jdt.internal.launching.environments package in order to simplify it.

Author checklist

• [ ] I have thoroughly tested my changes
• [x] The change is following the coding conventions
• [x] I have signed the Eclipse Contributor Agreement (ECA)

[HannesWell]

@iloveeclipse is there anything left that should be adjusted or is this ready for submission?