Remove @Keep from ExtractMarker class.

The main method is already specified as kept in src/main/keep.txt

Change-Id: I1da2180ef3972a988fa81423705c6777d143eac4
diff --git a/src/main/java/com/android/tools/r8/ExtractMarker.java b/src/main/java/com/android/tools/r8/ExtractMarker.java
index bda3597..0b1fead 100644
--- a/src/main/java/com/android/tools/r8/ExtractMarker.java
+++ b/src/main/java/com/android/tools/r8/ExtractMarker.java
@@ -26,7 +26,6 @@
 import java.util.Collection;
 import java.util.concurrent.ExecutionException;
 
-@Keep
 public class ExtractMarker {
   public static class VdexOrigin extends Origin {