clean SVG files

doc/eva/gui-images/RMB_click.svg

doc/eva/gui-images/gui-callstacks.svg

doc/eva/gui-images/gui-values.svg

share/frama-c-icon.svg

share/theme/flat/considered_valid.svg

share/theme/flat/fold.svg

share/theme/flat/inconsistent.svg

share/theme/flat/invalid_but_dead.svg

share/theme/flat/invalid_under_hyp.svg

share/theme/flat/never_tried.svg

share/theme/flat/surely_invalid.svg

share/theme/flat/surely_valid.svg

share/theme/flat/unfold.svg

share/theme/flat/unknown.svg

share/theme/flat/unknown_but_dead.svg

share/theme/flat/valid_but_dead.svg

share/theme/flat/valid_under_hyp.svg