resolve lint error for register_email_layout.xml (#163)

Author: wuman

auth/src/main/res/layout/register_email_layout.xml

@@ -4,7 +4,7 @@
     android:layout_height="match_parent"
     android:layout_width="match_parent">
 
-    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    <RelativeLayout
                   style="@style/FirebaseUI.WrapperStyle"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content">