remove threeten from executor lib

Author: diegomarquezp

google-cloud-spanner-executor/pom.xml

@@ -145,10 +145,6 @@
       <groupId>com.google.api</groupId>
       <artifactId>gax-grpc</artifactId>
     </dependency>
-    <dependency>
-      <groupId>org.threeten</groupId>
-      <artifactId>threetenbp</artifactId>
-    </dependency>
     <dependency>
       <groupId>com.google.code.findbugs</groupId>
       <artifactId>jsr305</artifactId>