[roottest] Remove JsMVA test ouput files

The JsMVA tests produce some output files with the trained models as a
side effect, and these files were unnecessarily added to the repository
[1].

Note: the JsMVA tests have been disabled since 2017 [2], and JsMVA was
excluded from the builds since ROOT 6.38 [3].

[1] root-project/roottest@03a6250
[2] root-project/roottest@387d707
[3] #19040

Author: guitargeek