This change removes usages of global Gerrit.KeyboardShortcutBinder and Gerrit.KeyboardShortcutBehavior and replaces it with direct import. Also, the KeyboardShortcutMixin is removed, because it was introduced as a workaround for polylint tests and is no longer needed. Change-Id: I4ed249efacd7b06dd2587411d982e98ebe81dfe6