Remove references to AR::Observer from docs

ActiveRecord::Observer was extracted into a separate gem so it
should not be referenced anymore (see https://github.com/rails/rails/commit/ccecab3)

1 files changed, 0 insertions, 8 deletions

diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index 401e6f0596..e6f2db2a2d 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -2067,14 +2067,6 @@ The sum of an empty receiver can be customized in this form as well:
 [].sum(1) {|n| n**3} # => 1
 ```
 
-The method `ActiveRecord::Observer#observed_subclasses` for example is implemented this way:
-
-```ruby
-def observed_subclasses
-  observed_classes.sum([]) { |klass| klass.send(:subclasses) }
-end
-```
-
 NOTE: Defined in `active_support/core_ext/enumerable.rb`.
 
 ### `index_by`