AbstractUseBuilder: Remove unused mapAstUse call

AbstractUseBuilder was calling editor()->parseSession()->mapAstUse()
on the builder's parent class, expecting the language-defined context
builder to have all that. It seems this was only used by the oldcpp plugin;
all other existing language plugins that use AbstractUseBuilder have dummy
implementations of editor(), mapAstUse(), and m_mapAst just to make the
code compile. So let's just remove it.