drop broken pinning mechanism

OpenKeychain/src/main/res/layout/add_keyserver_dialog.xml

@@ -53,11 +53,4 @@
         android:checked="true"
         android:text="@string/label_verify_keyserver_connection" />
 
-    <CheckBox
-        android:id="@+id/only_trusted_keyserver_checkbox"
-        android:layout_width="wrap_content"
-        android:layout_height="wrap_content"
-        android:checked="true"
-        android:text="@string/label_only_trusted_keyserver" />
-
 </LinearLayout>