[pom] Use empty relativePath

To prevent maven from issuing warnings if upper directory has a pom that
is unrelated.  The parent shouldn't be checked there but rather in .m2
first.

maven-wrapper/pom.xml

@@ -4,6 +4,7 @@
     <groupId>io.takari</groupId>
     <artifactId>takari</artifactId>
     <version>25</version>
+    <relativePath />
   </parent>
   
   <artifactId>maven-wrapper</artifactId>