Maven binaries should be cleared if necessary

[snicoll]

Running ./gradlew clean in spring-boot-project/spring-boot-tools/spring-boot-maven-plugin does not seem to clear prepared Maven binaries for integration tests.

As a result, when older versions are made out of scope, local tests still run against them as build/maven-binaries is not cleaned.

[wilkinsona]

There are a couple of things:

1. clean needs to wipe out the maven-binaries directory
2. clean should not be necessary as the contents of the maven-binaries directory should be kept in sync with the configured binaries that we're testing against.

[wilkinsona]

clean needs to wipe out the maven-binaries directory

As far as I can tell, this already happens. We need to address 2 though.