build.gradle: remove project.ext block
build.gradle: remove project.ext block

The only project property inside, `libDir`, is not used in the build
script at all.

It is also unlikely that we would be using project properties in the
future. They are only required when we have multi-project build, and the
root project needs to share properties with its child projects[1]. In
most other cases, local variables, scoped to the location where they are
meaningful, would be more appropriate.

To simplify the build.gradle script, let's remove the project.ext block.

[1] See "Property and method inheritance" in
    https://docs.gradle.org/2.12/userguide/multi_project_builds.html

se-edu/addressbook-level3,damithc/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4