Remove mentions of "callback mode" from docs

The notion of callback mode was removed in v4, the remaining references to it in the documentation may confuse users.

Author: ericcornelissen

Diff for: docs/01-writing-tests.md

@@ -2,7 +2,7 @@
 
 Translations: [Français](https://github.com/avajs/ava-docs/blob/main/fr_FR/docs/01-writing-tests.md)
 
-Tests are run concurrently. You can specify synchronous and asynchronous tests. Tests are considered synchronous unless you return a promise, an [observable](https://github.com/zenparsing/zen-observable), or declare it as a callback test.
+Tests are run concurrently. You can specify synchronous and asynchronous tests. Tests are considered synchronous unless you return a promise or an [observable](https://github.com/zenparsing/zen-observable).
 
 You must define all tests synchronously. They can't be defined inside `setTimeout`, `setImmediate`, etc.
 
@@ -73,8 +73,6 @@ test('promises the truth', async t => {
 
 AVA comes with built-in support for [observables](https://github.com/zenparsing/es-observable). If you return an observable from a test, AVA will automatically consume it to completion before ending the test.
 
-*You do not need to use "callback mode" or call `t.end()`.*
-
 ```js
 test('handles observables', t => {
 	t.plan(3);

Diff for: docs/recipes/test-setup.md

@@ -16,7 +16,7 @@ You could do all these things using plain setup functions, but there are tradeof
 |---|---
 | ⛔️   used for all tests| ✅   can change or skip depending on test
 | ⛔️   more overhead for beginners, "some magic"| ✅   easier for beginners, "no magic"
-| ✅   supports callback mode, built-in support for observables| ⛔️   must use promises for asynchronous behavior
+| ✅   built-in support for observables| ⛔️   must use promises for asynchronous behavior
 | ✅   failure has friendly output| ⛔️   errors are attributed to the test
 | ✅   corresponding `afterEach` and `afterEach.always` for cleanup| ⛔️   cannot easily clean up