Revert "Swap dependency to buildgraph-view"

This reverts commit 952baacafd3660a5da5cc13235457107d4eece5d.
See pull request #50 as well as jenkinsci/buildgraph-view#12

In hindsight, there is nothing wrong with build-flow plugin to consume
extension points from buildgraph-view if there are available. That
is exactly why the dependency is optional.
[maven-release-plugin] prepare release build-flow-plugin-0.17