Fix Python documentation for range type fields

GDB's Python documentation claims that range types have two fields,
but this is not true, and attempts to access them hit this error:

      "Type is not a structure, union, enum, or function type."

This patch fixes the documentation.

Author: tromey

gdb/doc/python.texi

@@ -1403,9 +1403,6 @@ Return the fields of this type.  The behavior depends on the type code:
 @item
 For structure and union types, this method returns the fields.
 
-@item
-Range types have two fields, the minimum and maximum values.
-
 @item
 Enum types have one field per enum constant.