[RF] Fix TTree ownership problem.
[RF] Fix TTree ownership problem.

[ROOT-10475] After writing a RooDataSet, the internal TTree is owned
both by the TFile and the RooFit object. This leads to a double delete
when the file is closed.

olifre/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,root-mirror/root