doc: remove out-of-date matter from internal docs

Remove out-of-date stability index material from `tools/doc/README.md`.
The stability index is documented in `doc/api/documentation.markdown`.

PR-URL: #5421
Reviewed-By: Evan Lucas <evanlucas@me.com>

tools/doc/README.md

@@ -69,28 +69,3 @@ Each type of heading has a description block.
 * Events have (list of arguments, description)
 * Methods have (list of arguments, description)
 * Properties have (type, description)
-
-## Stability ratings: 0-5
-
-These can show up below any section heading, and apply to that section.
-
-0 - Deprecated.  This feature is known to be problematic, and changes are
-planned.  Do not rely on it.  Use of the feature may cause warnings.  Backwards
-compatibility should not be expected.
-
-1 - Experimental.  This feature was introduced recently, and may change
-or be removed in future versions.  Please try it out and provide feedback.
-If it addresses a use-case that is important to you, tell the node core team.
-
-2 - Unstable.  The API is in the process of settling, but has not yet had
-sufficient real-world testing to be considered stable. Backwards-compatibility
-will be maintained if reasonable.
-
-3 - Stable.  The API has proven satisfactory, but cleanup in the underlying
-code may cause minor changes.  Backwards-compatibility is guaranteed.
-
-4 - API Frozen.  This API has been tested extensively in production and is
-unlikely to ever have to change.
-
-5 - Locked.  Unless serious bugs are found, this code will not ever
-change.  Please do not suggest changes in this area, they will be refused.