Update USAGE document microlibrary to filter out deprecated relationships appropriately

[jondricek]

Fixes #12

As far as I can tell, this should address the microlibrary issue in the USAGE.md file

[isaisabel]

Overall looks good. I have one possible improvement to the imports on the microlibrary, but that's very much an optional improvement so I'm marking this as "approved."

One other thing to note: the microlibrary is also included in the USAGE document on MITRE/CTI. We had apparently fixed this bug already over there (and forgotten to propagate the fix to this copy of the USAGE), but we might want to update that document so that they have the same implementation, especially since your solution is cleaner than what we have on MITRE/CTI.

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We also should consider the long term maintenance plan of the two usage documents, the fact they're maintained in parallel means it's easy to accidentally forget to propagate a correction to both.

We may want to remove the MITRE/CTI USAGE document and replace it with a link to this document so we don't have to maintain two in parallel. This instance of the document includes information about the ATT&CK Spec features not available in the STIX 2.0 dataset so they're not completely identical, but the STIX 2.1 additions are clearly marked so it should be OK to remove the MITRE/CTI USAGE in favor of a single document on this repo:

[jondricek]

As you already saw, I had added an issue on the MITRE/cti repo about keeping the two documents in sync. I like the idea of maintaining a single document here and just referencing it from the MITRE/cti repo. Given your approval, I'll go ahead and merge this PR now.