remove "Complete key exchanges" from advanced options

Closes #2664

//FREEBIE

res/xml/preferences_advanced.xml

@@ -16,11 +16,6 @@
                 android:title="@string/preferences__choose_identity"
                 android:summary="@string/preferences__choose_your_contact_entry_from_the_contacts_list"/>
 
-    <CheckBoxPreference android:defaultValue="true"
-                        android:key="pref_auto_complete_key_exchange"
-                        android:title="@string/preferences__complete_key_exchanges"
-                        android:summary="@string/preferences__automatically_complete_key_exchanges_for_new_sessions_or_for_existing_sessions_with_the_same_identity_key" />
-
     <Preference android:key="pref_submit_debug_logs"
                 android:title="@string/preferences__submit_debug_log"/>
 </PreferenceScreen>