Describe the bug
I'm given an error that I believe incorrectly interprets the GTFS reference document on shapes.txt. The error claims that all shapes defined must be used, which I don't believe is true.

To Reproduce
I'm running this on Tuolumne's October GTFS file, which is not available online, but attached here:
tuolumne.zip

Expected behavior
I did not expect to see this error message because I don't see this requirement in Google's reference document.

Witnessed behavior
Notice{filename='shapes.txt', level='ERROR', code='38', title='Unused shape', description='All records defined by GTFS shapes.txtshould be used intrips.txt. File shapes.txtdefines entity with id: 23that is not used.', extra='{fieldName=shape_id}'}]

Environment used

• OS: macOS Catalina 10.15.6
• Java version: 11.0.5
• gtfs-validator version: v1.3.0-SNAPSHOT