build.gradle: remove unused headless system property
build.gradle: remove unused headless system property

java.awt.robot is unused. To verify, download openjdk9 [1] and perform
`grep -r 'java\.awt\.robot' .` in that directory. There will be no
results, indicating that java.awt.robot is unused in openjdk9.
Therefore, setting java.awt.robot to true in build.gradle does not do
anything.

Let's remove it.

[1]: http://jdk.java.net/9/

se-edu/addressbook-level3