Ignore deleted files in `view_to_js_mappings.rb`

What does this MR do and why?

Ignore non-existing changed files in view_to_js_mappings.rb because it can be deleted

This fixes !107996 (comment 1236793234)

Note that this is just a quick fix for the error. We likely need to read from the deleted files to detect js- attributes there to see if it can affect existing JavaScript files, because they might be still referring to non-existing attributes. Follow up issue: #387936

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