Issue/custom code removal

Drive-by simplification of CompilerSettings.

I vow that one day we'll make this a FieldEditorPreferencePage and remove all that cruft.
What is the subject of sentences "return the value set in the preference store", "return the value in the UI" ? i.e. what function are you referring to ? 

If you mean `isUseEnabled`, please note the (latest) definition of `getValue`.