This reverts commit e99d2199d8b613f6099eb3fd577b9700b4050b8f.
The main spring-boot project should not have compile scope dependencies
to any org.slf4j artifacts.
Rework main build POM to be an aggregator pom that does not inherit
from any parent. Introduce new spring-boot-dependencies module to
act as a parent for both spring-boot-starter-parent and
spring-boot-parent.