drop configJson

The initial idea was to store exactly where the backups are stored.
But this only causes problems for migrations where the bucket might
change and clones where the prefix (box.id) changes.

Thus, it's best to leave the url creation to the caas side. (That
has to be done in another change)

migrations/schema.sql

@@ -105,6 +105,5 @@ CREATE TABLE IF NOT EXISTS backups(
     type VARCHAR(16) NOT NULL, /* 'box' or 'app' */
     dependsOn VARCHAR(4096), /* comma separate list of objects this backup depends on */
     state VARCHAR(16) NOT NULL,
-    configJson TEXT, /* configuration - bucket, prefix, key, provider */
 
     PRIMARY KEY (filename));