Remove double-import of settings

Causing warnings with Closure Compiler

External copy of cl/222162967

lib/mixins/element-mixin.js

@@ -9,14 +9,13 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 */
 import '../utils/boot.js';
 
-import { rootPath, strictTemplatePolicy, allowTemplateFromDomModule } from '../utils/settings.js';
+import { rootPath, strictTemplatePolicy, allowTemplateFromDomModule, legacyOptimizations } from '../utils/settings.js';
 import { dedupingMixin } from '../utils/mixin.js';
 import { stylesFromTemplate, stylesFromModuleImports } from '../utils/style-gather.js';
 import { pathFromUrl, resolveCss, resolveUrl } from '../utils/resolve-url.js';
 import { DomModule } from '../elements/dom-module.js';
 import { PropertyEffects } from './property-effects.js';
 import { PropertiesMixin } from './properties-mixin.js';
-import { legacyOptimizations } from '../utils/settings.js';
 
 /**
  * Current Polymer version in Semver notation.