Fix duplicate <dependencyManagement /> tag in BOM

[mkurz]

Publishing a first experimental milestone for the next Play major version didn't work.

So, the BillOfMaterialsPlugin plugin appends the dependency listings to pomExtra. Unfortunaly, until now, we did the same as well., so the entries were duplicated.
Until now however this didn't cause any problems, because interplay "hard reseted" pomExtra, so it kind of overrode one of those settings. Since last interplay release however it does not set pomExtra anymore, so now both settings kick in, creating the duplicates.
(This problem did not materialize in Play 2.8.x because it uses an old version of interplay).

I also think publishTo shouldn't be set here because the sbt-ci-release plugin already handles that.

[mkurz]

Not necessary to wait for tests, but I want to see the snaphot that gets published.