doc: remove Locked from stability index

The stability index 3 (Locked) is unused and is being eliminated. Remove
it from the documentation about the stability index.

Remove mention of the Locked from CONTRIBUTING.md. The remaining text
about the stability index is slight and not seemingly valuable. Removing
it too.

Refs: #11200

CONTRIBUTING.md

@@ -46,15 +46,6 @@ $ git remote add upstream git://github.com/nodejs/node.git
 For developing new features and bug fixes, the `master` branch should be pulled
 and built upon.
 
-#### Respect the stability index
-
-The rules for the master branch are less strict; consult the
-[stability index](./doc/api/documentation.md#stability-index) for details.
-
-In a nutshell, modules are at varying levels of API stability. Bug fixes are
-always welcome but API or behavioral changes to modules at stability level 3
-(Locked) are off-limits.
-
 #### Dependencies
 
 Node.js has several bundled dependencies in the *deps/* and the *tools/*

doc/api/documentation.md

@@ -56,12 +56,6 @@ The API has proven satisfactory. Compatibility with the npm ecosystem
 is a high priority, and will not be broken unless absolutely necessary.
 ```
 
-```txt
-Stability: 3 - Locked
-Only bug fixes, security fixes, and performance improvements will be accepted.
-Please do not suggest API changes in this area; they will be refused.
-```
-
 ## JSON Output
 
 > Stability: 1 - Experimental