Fix damaging schema

Remove forcing defaultValue on directiveMap generation because it leads to adding redundant defaultValue field to Scalars itself into typeMap:

{
    "typeMap": {
        "Int": {
            "defaultValue": 1, <- ERROR
            "parseLiteral": "function: 0x415a8fe8",
            "__type": "Scalar",
            "nonNull": {},
            "serialize": "function: 0x415a8ed8",
            "isValueOfTheType": "function: 0x415a8e98",
            "name": "Int",
            "description": "..."
        }
    }
}

Author: no1seman

graphql/schema.lua

@@ -117,7 +117,6 @@ function schema:generateDirectiveMap()
           if argumentType == nil then
             error('Must supply type for argument "' .. name .. '" on "' .. directive.name .. '"')
           end
-          argumentType.defaultValue = argument.defaultValue
           self:generateTypeMap(argumentType)
       end
     end