doc: remove legacy mode deprecation in assert

The deprecation was revoked in
#28892.

PR-URL: #28909
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/assert.md

@@ -116,8 +116,6 @@ This will also deactivate the colors in the REPL.
 
 ## Legacy mode
 
-> Stability: 0 - Deprecated: Use strict mode instead.
-
 Legacy mode uses the [Abstract Equality Comparison][] in:
 
 * [`assert.deepEqual()`][]