Remove calls to deprecated empty ClassImp macro

[ferdymercury]

and also change a couple of deprecated ClassDefT to ClassDef

[github-actions]

Test Results

    20 files      20 suites   3d 20h 53m 50s ⏱️
 3 663 tests  3 661 ✅ 0 💤 2 ❌
71 547 runs  71 539 ✅ 6 💤 2 ❌

For more details on these failures, see this check.

Results for commit 0c9d562e.

[guitargeek]

Thank you very much!

I did this already for RooFit before the 6.36 release, where it has caused no problems at all:

• #17639

So removing these macros from the whole codebase as a next step makes sense to me.