Move the piston-image crate from primary to stable

Author: rylev

collector/benchmarks/README.md

@@ -36,7 +36,6 @@ They mostly consist of real-world crates.
 - **html5ever**: An HTML parser. Stresses macro parsing code significantly.
 - **hyper-2**: A fairly large crate. Utilizes async/await, and used by
   many Rust programs.
-- **piston-image**: A modular game engine. An interesting Rust program.
 - **regex-1.5.5**: A regular expression parser. Used by many Rust programs.
 - **ripgrep**: A line-oriented search tool. A widely-used utility.
 - **ripgrep-13.0.0**: A line-oriented search tool. A widely-used utility.
@@ -141,7 +140,7 @@ Rust code being written today.
   a very large function containing many locals and basic blocks, similar to
   `keccak` but less extreme.
 - **regex**: See above. This is an older version of the crate.
-- **piston-image**: See above.
+- **piston-image**: See above. This is an older version of the `image` crate.
 - **style-servo**: An old version of Servo's `style` crate. A large crate, and
   one used by old versions of Firefox.
 - **syn**: See above.

collector/benchmarks/piston-image/perf-config.json

@@ -1,4 +1,4 @@
 {
     "runs": 1,
-    "category": "primary-and-stable"
-}
+    "category": "stable"
+}