Remove setting BP_NATIVE_IMAGE environment variable when building images

[scottfrederick]

When paketo-buildpacks/spring-boot#273 is implemented, Paketo buildpacks will build an image containing a native executable by default when invoked with a Spring Boot jar or war, there's no need for the Spring Boot Maven and Gradle plugins to set BP_NATIVE_IMAGE to true explicitly.

[mhalbritter]

We now no longer set BP_NATIVE_IMAGE: true. We, however, add Spring-Boot-Native-Processed: true to the manifest. This is read by the Spring Boot buildpack, which then provides the native-image-application buildplan entry, which triggers the native image buildpack and builds a native image.