Tags checker doesn't ignore deleted tags

Normally if tag does not exist in reference field (source or target at import time) we show an error about that and don't allow to save such segment.

Current behavior:
But in case trackchanges is enabled tag is actually not deleted from the editor, but wrapped in `del` tag, so it is not possible to save such segment with trackchanges enabled.

ExpectedBehavior:
Segment should be allowed to be saved in case excess tags are deleted.