Improved the AST reflection

[JohannesMeierSE]

This is a follow-up for #1942:

• removed outdated left-over code
• Another interesting change of Refactored AST reflection #1942 is documented by a new test case: meta-data are now generated even for types without properties
• The meta-data contained hard-coded string values ('MyType') and constants (MyType.$type): Now constants are used everywhere

[JohannesMeierSE]

Some more possible improvements are discussed in #1970 as future work

[spoenemann]

Thanks!