Marker.Id must now always be valid (if present).

src/main/java/de/dennisguse/opentracks/content/data/Marker.java

@@ -27,8 +27,6 @@ import androidx.annotation.VisibleForTesting;
 
 import java.util.Objects;
 
-import de.dennisguse.opentracks.util.PreferencesUtils;
-
 /**
  * @author Leif Hendrik Wilden
  * @author Rodrigo Damazio
@@ -180,11 +178,6 @@ public final class Marker {
             return id;
         }
 
-        @Deprecated //TODO Use a Id of null instead
-        public boolean isValid() {
-            return id != PreferencesUtils.RECORDING_TRACK_ID_DEFAULT;
-        }
-
         @Override
         public boolean equals(Object o) {
             if (this == o) return true;