Followup bd49f97: prevent MWExtensionNodes from having children

MWBlockExtensionNodes would duplicate their children in the output DOM:
they would appear once as restored from originalDomElements, and again
because the contents then got converted to linear model data and
that got converted back to DOM as well.

Bug: 66396
Change-Id: I4513ffc1cf3eb8726ac94252fe1526fac6d8bdf3