doc: remove extraneous paragraph from assert doc

The stability index is explained elsewhere in the documentation. It is
not necessary to repeat the information about Locked stability index in
the assert documentation.

PR-URL: #11174
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/api/assert.md

@@ -8,10 +8,6 @@ used in application code via `require('assert')`. However, `assert` is not a
 testing framework, and is not intended to be used as a general purpose assertion
 library.
 
-The API for the `assert` module is [Locked][]. This means that there will be no
-additions or changes to any of the methods implemented and exposed by
-the module.
-
 ## assert(value[, message])
 <!-- YAML
 added: v0.5.9
@@ -508,7 +504,6 @@ assert.throws(myFunction, 'missing foo', 'did not throw with expected message');
 assert.throws(myFunction, /missing foo/, 'did not throw with expected message');
 ```
 
-[Locked]: documentation.html#documentation_stability_index
 [`assert.deepEqual()`]: #assert_assert_deepequal_actual_expected_message
 [`assert.deepStrictEqual()`]: #assert_assert_deepstrictequal_actual_expected_message
 [`assert.ok()`]: #assert_assert_ok_value_message