Remove documentation about rules that are now enforced by RuboCop

RuboCop is _living_ documentation! ✨

[ci skip]

doc/development/testing.md

@@ -188,16 +188,10 @@ Please consult the [dedicated "Frontend testing" guide](./fe_guide/testing.md).
 ### General Guidelines
 
 - Use a single, top-level `describe ClassName` block.
-- Use `described_class` instead of repeating the class name being described
-  (_this is enforced by RuboCop_).
 - Use `.method` to describe class methods and `#method` to describe instance
   methods.
 - Use `context` to test branching logic.
-- Use multi-line `do...end` blocks for `before` and `after`, even when it would
-  fit on a single line.
 - Don't assert against the absolute value of a sequence-generated attribute (see [Gotchas](gotchas.md#dont-assert-against-the-absolute-value-of-a-sequence-generated-attribute)).
-- Don't supply the `:each` argument to hooks since it's the default.
-- Prefer `not_to` to `to_not` (_this is enforced by RuboCop_).
 - Try to match the ordering of tests to the ordering within the class.
 - Try to follow the [Four-Phase Test][four-phase-test] pattern, using newlines
   to separate phases.