remove unstable flag from 1.7 release notes

While this is technically available on the stable compiler in 1.7, we're planning on 'fixing' that in future versions, so let's not mention it here

Author: steveklabnik

RELEASES.md

@@ -89,9 +89,6 @@ Libraries
 Misc
 ----
 
-* [The `--error-format=json` flag to `rustc` causes it to emit errors
-  in JSON format][1.7j]. This is an unstable flag and so also requires
-  the `-Z unstable-options` flag.
 * [When running tests with `--test`, rustdoc will pass `--cfg`
   arguments to the compiler][1.7dt].
 * [The compiler is built with RPATH information by default][1.7rpa].
@@ -135,7 +132,6 @@ Compatibility Notes
 [1.7dta]: https://github.com/rust-lang/rust/pull/30394
 [1.7f]: https://github.com/rust-lang/rust/pull/30672
 [1.7h]: https://github.com/rust-lang/rust/pull/30818
-[1.7j]: https://github.com/rust-lang/rust/pull/30711
 [1.7ll]: https://github.com/rust-lang/rust/pull/30663
 [1.7m]: https://github.com/rust-lang/rust/pull/30381
 [1.7p]: https://github.com/rust-lang/rust/pull/30681