backups: remove noop backend

the noop backend is migrated into 0 sites config.

when the updater code sees that there is no site to backup, it will
just fail. user has to manually update with skipBackup flag.

src/backupformats.js

@@ -19,12 +19,9 @@ function api(format) {
     throw new BoxError(BoxError.INTERNAL_ERROR, `Undefined format ${format}`);
 }
 
-function validateFormat(provider, format) {
-    assert.strictEqual(typeof provider, 'string');
+function validateFormat(format) {
     assert.strictEqual(typeof format, 'string');
 
-    if (provider === 'noop') return null;
-
     if (format === 'tgz' || format == 'rsync') return null;
 
     return new BoxError(BoxError.BAD_FIELD, 'Invalid backup format');