build,doc: remove outdated lint-md-build

- In release guide
- In Travis config

Refs: https://github.com/nodejs/node/pull/20109

PR-URL: https://github.com/nodejs/node/pull/22991
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

.travis.yml

@@ -8,10 +8,8 @@ matrix:
   include:
     - name: "Linter"
       node_js: "latest"
-      env: 
+      env:
         - NODE=$(which node)
-      install:
-        - make lint-md-build
       script:
         - make lint
     - name: "Test Suite"

doc/releases.md

@@ -455,8 +455,8 @@ Cherry-pick the release commit to `master`. After cherry-picking, edit
 previously on `master`. `NODE_VERSION_IS_RELEASE` should be `0`. **Do not**
 cherry-pick the "Working on vx.y.z" commit to `master`.
 
-Run `make lint-md-build; make lint` before pushing to `master`, to make sure the
-Changelog formatting passes the lint rules on `master`.
+Run `make lint` before pushing to `master`, to make sure the Changelog
+formatting passes the lint rules on `master`.
 
 ### 13. Promote and Sign the Release Builds