build: remove deprecated and unused truth-java8-extension from invoker/core (#404)

Change-Id: I3a6965dd1a71f49b8b4389d2b6aea908131d9b1a

Co-authored-by: Anshul Patni <anshulpatni@google.com>

Author: AnshulPatni

invoker/core/pom.xml

@@ -139,12 +139,6 @@
       <version>1.4.5</version>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>com.google.truth.extensions</groupId>
-      <artifactId>truth-java8-extension</artifactId>
-      <version>1.4.5</version>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.eclipse.jetty</groupId>
       <artifactId>jetty-client</artifactId>