Remove obsolete warning about regular expression

This warning became obsolete when the regular expression was changed to
use `\z` instead of `\Z` in fce9c4e5e1.

    "-1234\n" =~ /\A[+-]?\d+\Z/ => 0
    "-1234\n" =~ /\A[+-]?\d+\z/ => nil

[ci skip]

guides/source/active_record_validations.md

@@ -490,9 +490,6 @@ If you set `:only_integer` to `true`, then it will use the
 regular expression to validate the attribute's value. Otherwise, it will try to
 convert the value to a number using `Float`.
 
-WARNING. Note that the regular expression above allows a trailing newline
-character.
-
 ```ruby
 class Player < ApplicationRecord
   validates :points, numericality: true