Remove extraneous case condition

In commit 72300f9 the code for the `String` condition was simplified, and it's
now the same as the code for the `*PERMITTED_TYPES` condition, and `String` is
one of the `PERMITTED_TYPES` so we can just eliminate this condition and let
`*PERMITTED_TYPES` cover it.

Author: jordan-brough

activejob/lib/active_job/arguments.rb

@@ -110,8 +110,6 @@ def serialize_argument(argument)
 
       def deserialize_argument(argument)
         case argument
-        when String
-          argument
         when *PERMITTED_TYPES
           argument
         when BigDecimal # BigDecimal may have been legacy serialized; Remove in 7.2