file has since been removed from content (#453)

Should of been part of 2ef961e

Author: tshepang

src/SUMMARY.md

@@ -23,7 +23,6 @@
     - [Coding conventions](./conventions.md)
     - [crates.io Dependencies](./crates-io.md)
     - [Emitting Errors and other Diagnostics](diagnostics.md)
-        - [JSON diagnostic format](diagnostics/json-format.md)
 - [Part 2: How rustc works](./part-2-intro.md)
     - [High-level overview of the compiler source](./high-level-overview.md)
     - [The Rustc Driver and Interface](./rustc-driver.md)