remove unnecessary impl

Author: igorbernstein2

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/rpc/FixedTransportChannelProvider.java

@@ -58,12 +58,6 @@ public boolean needsExecutor() {
     return false;
   }
 
-  @Nullable
-  @Override
-  public Executor getExecutor() {
-    return null;
-  }
-
   @Override
   public FixedTransportChannelProvider withExecutor(ScheduledExecutorService executor) {
     return withExecutor((Executor) executor);