Remove attributes from Argument annotation

Prior to this commit, the `@Argument` annotation would support two
attributes: `required` and `defaultValue`, specifying whether arguments
should be considered as required and default values to use if they're
not present.

This can be problematic as this can already be specified at the schema
level.

To avoid duplication and invalid setups betweens schema and Controllers,
this commit removes those attributes in favor of the schema definition.

Closes gh-150

spring-graphql-docs/src/docs/asciidoc/index.adoc

@@ -577,8 +577,7 @@ values. The arguments are available as simple scalar values such as String, or a
 of values  for more complex input, or a `List` of values.
 
 Use `@Argument` to access an argument for the field that maps to the handler method. You
-can declare such a method parameter to be of any type. If necessary, Spring GraphQL
-converts the value by serializing it to JSON first and then to the target type.
+can declare such a method parameter to be of any type.
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
@@ -601,8 +600,8 @@ You can explicitly specify the argument name, for example `@Argument("bookInput"
 it not specified, it defaults to the method parameter name, but this requires the
 `-parameters` compiler flag with Java 8+ or debugging information from the compiler.
 
-By default, an `@Argument` is required, but you can make it optional by setting the
-`required` flag to false or by declaring the argument with `java.util.Optional`.
+The "required" character of an `@Argument` or its default value is controlled at the schema
+level.
 
 You can use `@Argument` on a `Map<String, Object>` argument, to obtain all argument
 values. The name attribute on `@Argument` must not be set.