Merge rust-bitcoin#1804: extended_tests: Remove stale docs

53d75c7 extended_tests: Remove stale docs (Tobin C. Harding)

Pull request description:

  We no longer need to pin `syn`; remove stale docs.

  This is a follow up to rust-bitcoin#1696

ACKs for top commit:
  Kixunil:
    ACK 53d75c7
  apoelstra:
    ACK 53d75c7

Tree-SHA512: 9e939860cfb3731942e92a611632f0b24acc88e06a96e905ee2ae50e489d1610f7b9c79e20384196482aad4d5da182f6f919d18726e079a98a667532806d5aae

Author: apoelstra

hashes/extended_tests/schemars/README.md

@@ -4,7 +4,6 @@ Run as usual with `cargo test`.
 
 ## Minimum Supported Rust Version (MSRV)
 
-To run the tests with the MSRV you will need to pin some dependencies:
+To run the tests with the MSRV you will need to pin `serde`:
 
 - `cargo update -p serde --precise 1.0.156`
-- `cargo update -p syn --precise 1.0.107`