Remove stale identity approval preference (no longer used)

Fixes #6745
// FREEBIE

res/xml/preferences_app_protection.xml

@@ -34,12 +34,6 @@
     <PreferenceCategory android:layout="@layout/preference_divider"/>
 
     <PreferenceCategory android:title="@string/preferences_app_protection__communication">
-        <org.thoughtcrime.securesms.components.SwitchPreferenceCompat
-                android:defaultValue="false"
-                android:key="pref_approve_identity_changes"
-                android:title="@string/preferences_app_protection__safety_numbers_approval"
-                android:summary="@string/preferences_app_protecting__require_approval_of_new_safety_numbers_when_they_change"/>
-
         <org.thoughtcrime.securesms.components.SwitchPreferenceCompat
                 android:defaultValue="false"
                 android:key="pref_turn_only"

src/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -79,7 +79,6 @@ public class TextSecurePreferences {
   private static final String UPDATE_APK_DIGEST                = "pref_update_apk_digest";
   private static final String SIGNED_PREKEY_ROTATION_TIME_PREF = "pref_signed_pre_key_rotation_time";
   private static final String IN_THREAD_NOTIFICATION_PREF      = "pref_key_inthread_notifications";
-  private static final String APPROVAL_IDENTITY_CHANGES_PREF   = "pref_approve_identity_changes";
   private static final String SHOW_INVITE_REMINDER_PREF        = "pref_show_invite_reminder";
   public  static final String MESSAGE_BODY_TEXT_SIZE_PREF      = "pref_message_body_text_size";
 
@@ -156,14 +155,6 @@ public class TextSecurePreferences {
     return getBooleanPreference(context, MULTI_DEVICE_PROVISIONED_PREF, false);
   }
 
-  public static boolean isSendingIdentityApprovalRequired(Context context) {
-    return getBooleanPreference(context, APPROVAL_IDENTITY_CHANGES_PREF, false);
-  }
-
-  public static void setSendingIdentityApprovalRequired(Context context, boolean value) {
-    setBooleanPreference(context, APPROVAL_IDENTITY_CHANGES_PREF, value);
-  }
-
   public static void setSignedPreKeyFailureCount(Context context, int value) {
     setIntegerPrefrence(context, SIGNED_PREKEY_FAILURE_COUNT_PREF, value);
   }