Ignore key mangadex_default in config YAML and remove unnecessary

calls to `YAML::Field`

src/config.cr

@@ -3,26 +3,17 @@ require "yaml"
 class Config
 	include YAML::Serializable
 
-	@[YAML::Field(key: "port")]
 	property port : Int32 = 9000
-
-	@[YAML::Field(key: "library_path")]
 	property library_path : String = \
 		File.expand_path "~/mango/library", home: true
-
-	@[YAML::Field(key: "db_path")]
 	property db_path : String = \
 		File.expand_path "~/mango/mango.db", home: true
-
 	@[YAML::Field(key: "scan_interval_minutes")]
 	property scan_interval : Int32 = 5
-
-	@[YAML::Field(key: "log_level")]
 	property log_level : String = "info"
-
-	@[YAML::Field(key: "mangadex")]
 	property mangadex = Hash(String, String|Int32).new
 
+	@[YAML::Field(ignore: true)]
 	@mangadex_defaults = {
 		"base_url" => "https://mangadex.org",
 		"api_url" => "https://mangadex.org/api",