Remove LearningResourceMetadataURI in favor of URI Type

Under 2.1 the LearningResourceMetadataURI has an identical definition to the URI type, but a different length 225 vs 255. proposal is to retire/remove this type in favor of just using the URI type. The differing length could potentially cause issues.

See:

http://schema.ed-fi.org/datahandbook-v21/MetaEd-Handbook-Index.html#/LearningResourceMetadataURI66

and 

http://schema.ed-fi.org/datahandbook-v21/MetaEd-Handbook-Index.html#/URI115