Fully remove the deprecated `assay_type` files and references

[yuanzhou]

Following the work of https://github.com/hubmapconsortium/search-api/pull/832, we'll need to fully remove the references to the old assay type handlings.

• Remove all files under search-api's src/libs
• Remove src/search-schema and all files inside. John Conroy confirmed.
• Confirm with Harvard how to handle the portal test scripts properly. There are being used by the top-level test.sh with Travis CI
• Remove the GETT /assaytype, POST /assayname, and GET /assaytype/<name> endpoints from Smart API yaml and AWS API gateway
• Remove the corresponding documentation from the docs repo