Get rid of Inset::setPosCache

Get rid of Inset::setPosCache

This helper method is used to set the inset position cache at drawing
time. However this is already done by the drawing code itself
(MathRow::draw, RowPainter::paintInset), so that there is no need to
do this same work again in a different place.

Note that the inset positions are still set in InsetTabular::draw for
each cell.

This commit comes with a mild risk attached (it might be that some of
these calls were useful after all!), but all of this is fixable.