Remove challenges table (#5285)

This table is not used by any Boulder logic nor by any
offline stats/metrics gathering.

sa/_db-next/migrations/20210211174900_DropTableChallenges.sql

@@ -0,0 +1,23 @@
+
+-- +goose Up
+-- SQL in section 'Up' is executed when this migration is applied
+
+DROP TABLE `challenges`;
+
+-- +goose Down
+-- SQL section 'Down' is executed when this migration is rolled back
+
+CREATE TABLE `challenges` (
+  `id` bigint(20) NOT NULL AUTO_INCREMENT,
+  `authorizationID` varchar(255) NOT NULL,
+  `LockCol` bigint(20) DEFAULT NULL,
+  `type` varchar(255) NOT NULL,
+  `status` varchar(255) NOT NULL,
+  `error` mediumblob DEFAULT NULL,
+  `validated` datetime DEFAULT NULL,
+  `token` varchar(255) NOT NULL,
+  `validationRecord` mediumblob DEFAULT NULL,
+  `keyAuthorization` varchar(255) DEFAULT NULL,
+  PRIMARY KEY (`id`),
+  KEY `authorizationID_challenges_idx` (`authorizationID`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;

sa/model.go

@@ -182,8 +182,6 @@ type challModel struct {
 
 	// TODO(#1818): Remove, this field is unused, but is kept temporarily to avoid a database migration.
 	Validated bool `db:"validated"`
-
-	LockCol int64
 }
 
 // newReg creates a reg model object from a core.Registration