Remove unused sequenceTask

Author: devversion

tools/gulp/gulpfile.ts

@@ -11,4 +11,4 @@ import './tasks/unit-test';
 import './tasks/aot';
 import './tasks/payload';
 import './tasks/coverage';
-import './tasks/library';
+import './tasks/library';

tools/gulp/tasks/clean.ts

@@ -1,6 +1,6 @@
 import {task} from 'gulp';
 import {DIST_ROOT} from '../constants';
-import {cleanTask, sequenceTask} from '../util/task_helpers';
+import {cleanTask} from '../util/task_helpers';
 
 
 /** Deletes the dist/ directory. */