Remove sign key exchange preference

Yet another setting that most users will never touch. Workaround for
those who would is to use a different identity key per device.

Let this be a sacrifice to the android settings design pattern gods.

res/xml/preferences.xml

@@ -66,11 +66,6 @@
                            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" />
 
-    <CheckBoxPreference android:defaultValue="true"
-                        android:key="pref_send_identity_key"
-                        android:summary="@string/preferences__sign_key_exchange_messages_with_identity_key"
-                        android:title="@string/preferences__sign_key_exchange" />
-
     <CheckBoxPreference android:defaultValue="false"
                         android:key="pref_timeout_passphrase"
                         android:summary="@string/preferences__forget_passphrase_from_memory_after_some_interval"

src/org/thoughtcrime/securesms/ApplicationPreferencesActivity.java

@@ -63,7 +63,6 @@ public class ApplicationPreferencesActivity extends PassphraseRequiredSherlockPr
   public static final String LED_BLINK_PREF                   = "pref_led_blink";
   public static final String LED_BLINK_PREF_CUSTOM            = "pref_led_blink_custom";
   public static final String IDENTITY_PREF                    = "pref_choose_identity";
-  public static final String SEND_IDENTITY_PREF               = "pref_send_identity_key";
   public static final String ALL_MMS_PERF                     = "pref_all_mms";
   public static final String PASSPHRASE_TIMEOUT_INTERVAL_PREF = "pref_timeout_interval";
   public static final String PASSPHRASE_TIMEOUT_PREF          = "pref_timeout_passphrase";

src/org/thoughtcrime/securesms/crypto/KeyExchangeMessage.java

@@ -115,9 +115,7 @@ public class KeyExchangeMessage {
   }
 	
   private static boolean includeIdentitySignature(int messageVersion, Context context) {
-    return IdentityKeyUtil.hasIdentityKey(context) && 
-      (messageVersion >= 1)                   &&
-      PreferenceManager.getDefaultSharedPreferences(context).getBoolean(ApplicationPreferencesActivity.SEND_IDENTITY_PREF, true);
+    return IdentityKeyUtil.hasIdentityKey(context) && (messageVersion >= 1);
   }