[import] Removed "element removed flag".

This addresses part of https://www.w3.org/Bugs/Public/show_bug.cgi?id=24003.
[import] Turns import tree to import list as a simplification.

Also, this clarifies the import list is a separated data structure
that should be tracked separately from implicit tree structure
of import. This addresses unclarity pointed at
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24003