tutorial/015-Prerequisite: Remove pre-R7-release info

[njbartlett]

Until the OSGi R7 release has completed the enRoute archetypes and indexes must all have SNAPSHOT versions.

R7 has been released so this warning should not be necessary.

[timothyjward]

R7 has been released so this warning should not be necessary.

The APIs and specifications have been released, but the RIs and CTs won't be final for another few weeks. The lack of a released JAX-RS whiteboard implementation is currently blocking enRoute from being able to create a 7.0.0 release version.