Remove redundant groupId from pom.

The project groupId matches the parent groupId.
diff --git a/pom.xml b/pom.xml
index 07e4735..250fd89 100644
--- a/pom.xml
+++ b/pom.xml
@@ -24,7 +24,6 @@
     <version>50</version>
   </parent>
 
-  <groupId>org.apache.commons</groupId>
   <artifactId>commons-numbers-parent</artifactId>
   <version>1.0-SNAPSHOT</version>
   <packaging>pom</packaging>