Remove nullable annotation

Change-Id: I4325f0843d869cfbda6fe9e6ed8513670bfaabe9

src/main/java/com/android/tools/r8/utils/collections/ImmutableInt2ReferenceSortedMap.java

diff --git a/src/main/java/com/android/tools/r8/utils/collections/ImmutableInt2ReferenceSortedMap.java b/src/main/java/com/android/tools/r8/utils/collections/ImmutableInt2ReferenceSortedMap.java
index 69ba449..22bf012 100644
--- a/src/main/java/com/android/tools/r8/utils/collections/ImmutableInt2ReferenceSortedMap.java
+++ b/src/main/java/com/android/tools/r8/utils/collections/ImmutableInt2ReferenceSortedMap.java
@@ -13,7 +13,6 @@
 import java.util.Map;
 import java.util.function.BiFunction;
 import java.util.function.Function;
-import org.jetbrains.annotations.Nullable;
 
 public class ImmutableInt2ReferenceSortedMap<V> extends Int2ReferenceSortedMaps.EmptySortedMap<V> {
 
@@ -165,7 +164,6 @@
     throw new Unreachable("Should not modify an immutable structure");
   }
 
-  @Nullable
   @Override
   public V putIfAbsent(Integer key, V value) {
     throw new Unreachable("Should not modify an immutable structure");