Remove deprecate passing string to `:if` and `:unless` conditional options [ci skip]

Follow up to #27608

guides/source/active_record_callbacks.md

@@ -288,7 +288,7 @@ Article destroyed
 Conditional Callbacks
 ---------------------
 
-As with validations, we can also make the calling of a callback method conditional on the satisfaction of a given predicate. We can do this using the `:if` and `:unless` options, which can take a symbol, a string, a `Proc` or an `Array`. You may use the `:if` option when you want to specify under which conditions the callback **should** be called. If you want to specify the conditions under which the callback **should not** be called, then you may use the `:unless` option.
+As with validations, we can also make the calling of a callback method conditional on the satisfaction of a given predicate. We can do this using the `:if` and `:unless` options, which can take a symbol, a `Proc` or an `Array`. You may use the `:if` option when you want to specify under which conditions the callback **should** be called. If you want to specify the conditions under which the callback **should not** be called, then you may use the `:unless` option.
 
 ### Using `:if` and `:unless` with a `Symbol`
 
@@ -300,16 +300,6 @@ class Order < ApplicationRecord
 end
 ```
 
-### Using `:if` and `:unless` with a String
-
-You can also use a string that will be evaluated using `eval` and hence needs to contain valid Ruby code. You should use this option only when the string represents a really short condition:
-
-```ruby
-class Order < ApplicationRecord
-  before_save :normalize_card_number, if: "paid_with_card?"
-end
-```
-
 ### Using `:if` and `:unless` with a `Proc`
 
 Finally, it is possible to associate `:if` and `:unless` with a `Proc` object. This option is best suited when writing short validation methods, usually one-liners:

guides/source/active_record_validations.md

@@ -916,18 +916,6 @@ class Order < ApplicationRecord
 end
 ```
 
-### Using a String with `:if` and `:unless`
-
-You can also use a string that will be evaluated using `eval` and needs to
-contain valid Ruby code. You should use this option only when the string
-represents a really short condition.
-
-```ruby
-class Person < ApplicationRecord
-  validates :surname, presence: true, if: "name.nil?"
-end
-```
-
 ### Using a Proc with `:if` and `:unless`
 
 Finally, it's possible to associate `:if` and `:unless` with a `Proc` object