Reader Xls Dead Code

[oleibman]

Recent changes left Xls Reader with a ton of Scrutinizer messages. I have filtered away the false positives. Most of what is left is dead code, and this PR addresses those. Most of those are used to parse the BIFF data, but we either don't need to implement, or haven't yet implemented, those particular parsed data items. I think it is more useful to comment those out for documentary purposes rather than deleting them.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.