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

samples/webmvc-http/src/main/resources/application.properties

@@ -1,5 +1,3 @@
 management.endpoints.web.exposure.include=health,metrics,info
 
 spring.graphql.schema.printer.enabled=true
-
-spring.graph