internal: remove postversion script, update release docs

RELEASING.md

@@ -40,7 +40,7 @@
     ### Ship checklist
 
     - [ ] Update `CHANGELOG.md`
-    - [ ] Update the version in `package.json` with `npm version <version>`
+    - [ ] Update the `version` field in `package.json` to match the release version
     - [ ] [Create a new release](https://github.com/primer/css/releases/new)
     - [ ] [Update github/github](https://github.com/primer/css/blob/master/RELEASING.md#in-githubgithub)
     - [ ] Create a new pull request for the next release

package.json

@@ -30,7 +30,6 @@
     "lint-js": "eslint lib docs",
     "now-build": "next build",
     "now-start": "next start",
-    "postversion": "git commit -m \"chore: v$npm_package_version\" package*.json",
     "now-test": "npm-run-all -s now-build now-start",
     "postpublish": "script/postpublish",
     "prepare": "npm run dist",