Revert "Merge pull request #27686 from koic/friendly_bigdecimal_inspect"

The exact inspect output of a BigDecimal is out of scope for what we're trying
to communicate about `dup` and `duplicable?` here.

Adding two examples distracts is disctracting, so keep the docs from before
since our minimal version is Ruby 2.2.2.

[ Koichi ITO, Jon Moss, Kasper Timm Hansen ]

This reverts commit 2163874ded, reversing
changes made to 46fdbc5290.

activesupport/lib/active_support/core_ext/object/duplicable.rb

@@ -107,13 +107,6 @@ class BigDecimal
   # BigDecimals are duplicable:
   #
   # BigDecimal.new("1.2").duplicable? # => true
-  #
-  # In Ruby 2.4.0:
-  #
-  # BigDecimal.new("1.2").dup         # => 0.12e1
-  #
-  # Whereas in Ruby 2.2 and 2.3:
-  #
   # BigDecimal.new("1.2").dup         # => #<BigDecimal:...,'0.12E1',18(18)>
   def duplicable?
     true

guides/source/active_support_core_extensions.md

@@ -1980,15 +1980,7 @@ and that symbol specifiers are also supported:
 BigDecimal.new(5.00, 6).to_s(:db)  # => "5.0"
 ```
 
-Engineering notation is still supported.
-
-In Ruby 2.4:
-
-```ruby
-BigDecimal.new(5.00, 6).to_s("e")  # => "0.5e1"
-```
-
-Whereas in Ruby 2.2 and 2.3:
+Engineering notation is still supported:
 
 ```ruby
 BigDecimal.new(5.00, 6).to_s("e")  # => "0.5E1"