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: nodejs#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

@@ -5,10 +5,6 @@
 The `assert` module provides a simple set of assertion tests that can be used to
 test invariants.
 
-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
@@ -466,7 +462,6 @@ 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