Merge pull request #610 from adetaylor/edit-test-instructions

Removing obsolete test instructions.

CONTRIBUTING.md

@@ -37,22 +37,11 @@ tests for you.
 
 The test suite requires a nightly compiler.
 
-##### In the [`tests/deps`] directory
-
-```sh
-# This is a prerequisite for running the full test suite
-cargo clean && cargo update && cargo build
-```
-
-##### In the top level serde-json directory
-
 ```sh
 # Run the full test suite, including doc test and compile-tests
 cargo test
 ```
 
-[`tests/deps`]: https://github.com/serde-rs/json/tree/master/tests/deps
-
 ## Conduct
 
 In all Serde-related forums, we follow the [Rust Code of Conduct]. For