Remove sentence about the namespace of deprecation notification events

All Rails frameworks deprecators have "Rails" as their `gem_name`, and
the `:notify` behavior emits events based on that, so all frameworks
emit their deprecations under the `rails` namespace.

For example: activesupport/lib/active_support/deprecator.rb:5
activesupport/lib/active_support/deprecation.rb:42
activesupport/lib/active_support/deprecation/behaviors.rb:39

Author: etiennebarrie

guides/source/active_support_instrumentation.md

@@ -760,8 +760,6 @@ INFO. The only ActiveStorage service that provides this hook so far is GCS.
 | `:gem_name`            | Name of the gem reporting the deprecation             |
 | `:deprecation_horizon` | Version where the deprecated behavior will be removed |
 
-NOTE: Each framework will also emit their own namespaced `deprecation` events.
-
 Exceptions
 ----------