Fix warn polyfill when no uplevel is provided

An unspecified uplevel is not the same as an uplevel of 1:

```
$ irb
irb(main):001> warn("foo")
foo
=> nil
irb(main):002> warn("foo", uplevel: 1)
/home/user/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/irb-1.14.0/lib/irb/workspace.rb:121: warning: foo
=> nil
```

Author: Earlopain

lib/prism/polyfill/warn.rb

@@ -7,35 +7,25 @@
   Kernel.prepend(
     Module.new {
       def warn(*msgs, uplevel: nil, category: nil) # :nodoc:
-        uplevel =
-          case uplevel
-          when nil
-            1
-          when Integer
-            uplevel + 1
-          else
-            uplevel.to_int + 1
-          end
-
-        super(*msgs, uplevel: uplevel)
+        case uplevel
+        when nil
+          super(*msgs)
+        else
+          super(*msgs, uplevel: uplevel.to_int + 1)
+        end
       end
     }
   )
 
   Object.prepend(
     Module.new {
       def warn(*msgs, uplevel: nil, category: nil) # :nodoc:
-        uplevel =
-          case uplevel
-          when nil
-            1
-          when Integer
-            uplevel + 1
-          else
-            uplevel.to_int + 1
-          end
-
-        super(*msgs, uplevel: uplevel)
+        case uplevel
+        when nil
+          super(*msgs)
+        else
+          super(*msgs, uplevel: uplevel.to_int + 1)
+        end
       end
     }
   )