Remove Git plugin from Spring Boot tools

The Git plugin was primarily being used to provide version information
that Boot's maven plugin can add into the MANIFEST.MF of the uber-jars
that it creates under the Spring-Boot-Commit-Id attribute.

This commit removes the Git plugin from Boot's own projects, but
leaves it in the spring-boot-starter-parent for use by Spring
Boot-based applications.

The attribute in the uber-jars' MANIFEST.MF has been replaced with a
Spring-Boot-Version attribute. The value of this attribute is the
implementation version of Repackager class's package.