remove default key value

gax/src/main/java/com/google/api/gax/rpc/ApiCallContext.java

@@ -248,37 +248,26 @@ public interface ApiCallContext extends RetryingContext {
    * Return a new ApiCallContext with additional option merged into the present instance. Any
    * existing value of the key is overwritten.
    */
-  @BetaApi(
-      "The surface for api call context options is not stable yet and may change in the future.")
+  @BetaApi("The surface for call context options is not stable yet and may change in the future.")
   <T> ApiCallContext withOption(Key<T> key, T value);
 
   /** Return the api call context option set for this context. */
   @SuppressWarnings("unchecked")
-  @BetaApi(
-      "The surface for api call context options is not stable yet and may change in the future.")
+  @BetaApi("The surface for call context options is not stable yet and may change in the future.")
   <T> T getOption(Key<T> key);
 
   /** Key for api call context options key-value pair. */
   final class Key<T> {
     private final String name;
-    private final T defaultValue;
 
-    private Key(String name, T defaultValue) {
+    private Key(String name) {
       this.name = name;
-      this.defaultValue = defaultValue;
     }
 
-    /**
-     * Factory method for creating instances of {@link Key}. The default value of the key is null.
-     */
+    /** Factory method for creating instances of {@link Key}. */
     public static <T> Key<T> create(String name) {
       Preconditions.checkNotNull(name, "Key name cannot be null.");
-      return new Key<>(name, null);
-    }
-
-    /** Returns the user supplied default value of the key. */
-    public T getDefault() {
-      return defaultValue;
+      return new Key<>(name);
     }
   }
 }

gax/src/main/java/com/google/api/gax/rpc/internal/ApiCallContextOptions.java

@@ -76,7 +76,7 @@ public <T> ApiCallContextOptions withOption(Key<T> key, T value) {
   public <T> T getOption(Key<T> key) {
     Preconditions.checkNotNull(key);
     if (!options.containsKey(key)) {
-      return key.getDefault();
+      return null;
     }
     return (T) options.get(key);
   }