TMF: Remove XML header classes, they are not useful anymore

TMF: Remove XML header classes, they are not useful anymore

With the XmlUtils utility methods, there is no added value to wrap them in
a class and they are accessible from everywhere.

Change-Id: I66d15423ef4dadc18a77caef6af2d63faa426173
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/25584
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>