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.

spring-boot-tools/spring-boot-loader/src/main/resources/org/springframework/boot/git.properties

@@ -1,13 +0,0 @@
-#Generated by Git-Commit-Id-Plugin
-#Wed Oct 16 13:12:06 EDT 2013
-git.commit.id.abbrev=47cd5dd
-git.commit.user.email=dsyer@gopivotal.com
-git.commit.message.full=Tooling for PropertiesLauncher in JAR archives\n\nTo use PropertiesLauncher instead of JarLauncher in an\nexecutable JAR we have provided tooling support. In Maven\n(using the starter parent to default some of the settings)\:\n\n    <plugin>\n      <groupId>org.springframework.boot</groupId>\n      <artifactId>spring-boot-maven-plugin</artifactId>\n      <configuration>\n        <layout>ZIP</layout>\n      </configuration>\n    </plugin>\n\nin Gradle\:\n\n    apply plugin\: "spring-boot"\n    springBoot {\n        layout \= 'ZIP'\n      }\n    }\n\n[Fixes \#58837492] [bs-330] Add tooling for PropertiesLauncher\n
-git.commit.id=47cd5dd679ce10f95aaa795cdec7314189ab3c7c
-git.commit.message.short=Tooling for PropertiesLauncher in JAR archives
-git.commit.user.name=Dave Syer
-git.build.user.name=Dave Syer
-git.build.user.email=dsyer@gopivotal.com
-git.branch=master
-git.commit.time=2013-10-16T08\:49\:51-0400
-git.build.time=2013-10-16T13\:12\:06-0400