Remove JsMVA source code

[guitargeek]

The JsMVA features were not functional for years, and therefore excluded from ROOT 6.38 with commit 41f7ac4. Therefore, for ROOT 6.42, it's appropriate to remove the sources for good.

[dpiparo]

Thanks for this PR. Approved as per decision at the 200th PPP Meeting.

[github-actions]

Test Results

    23 files      23 suites   3d 16h 0m 29s ⏱️
 3 873 tests  3 873 ✅ 0 💤 0 ❌
79 694 runs  79 694 ✅ 0 💤 0 ❌

Results for commit 9ef8c24.

[hahnjo]

Not wanting to question the decision, but commit 41f7ac4 is then the reason that JsMVA didn't work at all for @hageboeck which means that all conclusions we jumped to during the PPP that it's entirely non-functional are our own cause.

[guitargeek]

Sorry, should have added more context: commit 41f7ac4 was made because it didn't work to begin with, and we didn't want to ship broken features.

If you try to run the tutorial that Stephan linked on slide 20 in the PPP talk in Swan with LCG 108 (ROOT 6.36.02, before commit 41f7ac4), it also fails at some point:

Same with ROOT 6.34 (I didn't check earlier releases though). If the one tutorial (that was not in the ROOT docs by the way) for a feature stopped working years ago, that's a strong signal that the feature is unused.

I suspect it didn't work correctly as far back as 2017, where the JsMVA unit tests were temporarily disabled.

[guitargeek]

Another data point: JsMVA used a pinned version of JsROOT that was never updated (see also #12441).

It was a bit awkward that ROOT itself uses different JsROOT versions because of this, so I looked into making JsMVA work with the current JsROOT. It turned out to be very challenging, because after all JsROOT history has continued for ~10 years... My conclusion was that for a feature that anyway didn't work, it's not worth to do this migration, and hence there was commit 41f7ac4.