Revert "refactor(server): move timestamp mixin to own module"

This reverts commit e8ba7d4.

Author: Walker Leite

meta.js

@@ -35,7 +35,6 @@ module.exports = {
     'server/initial-data/**/*': 'extended',
     'server/models/**/*': 'extended',
     'test/client/components/**/*': 'extended',
-    'test/server/mixins/**/*': 'extended',
     'test/server/account.spec.js': 'extended',
   },
   complete(data, {logger}) {

template/server/mixins/timestamp.js

@@ -9,6 +9,9 @@
     "mixins": [
       "loopback/common/mixins",
       "loopback/server/mixins",
+      {{#extended}}
+      "../node_modules/loopback-ds-timestamp-mixin",
+      {{/extended}}
       "../common/mixins",
       "./mixins"
     ]