Fix clang warnings

variable binds to a temporary

warning: loop variable ‘rootIndex’ of type ‘const QModelIndex&’ binds to a temporary constructed from type ‘const QPersistentModelIndex’

note: use non-reference type ‘const QModelIndex’ to make the copy explicit or ‘const QPersistentModelIndex&’ to prevent copying

unused parameter ‘count’

SimpleObjectModel::removeRows algorithms was wrong, it did not actually remove said number of objects. Fix it with a loop iterating from last to first row inclusive.