[Build] Fix warning about parent.relativePath in buildtools/pom.xml

[lhotari]

Motivation

The relativePath setting is invalid in buildtools/pom.xml.

Reproducing the issue:

$ cd buildtools
$ mvn verify
[INFO] Scanning for projects...
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.apache.pulsar:buildtools:jar:2.8.0-SNAPSHOT
[WARNING] 'parent.relativePath' of POM org.apache.pulsar:buildtools:2.8.0-SNAPSHOT (/home/lari/workspace-pulsar/pulsar/buildtools/pom.xml) points at org.apache.pulsar:pulsar instead of org.apache:apache, please verify your project structure @ line 25, column 11
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING] 
...

Modifications

Set relativePath to an empty string. This solution is also recommended in this StackOverflow answer: https://stackoverflow.com/a/6006098 .

[lhotari]

/pulsarbot run-failure-checks