update configuration to deal with pgun regardless of detector configuration (@adavis)

Merged Marco Cattaneo requested to merge cherry-pick-1f83fe3a into 2018-patches

With removal of RICH from standard sequences, productions for 2017 and 2018 data with PGun are not possible using current options. Remove reliance on externally setting the pGun reconstruction sequence and instead remove "Vertex" from internally configured reco sequence if pGun is passed.

Cherry-picked from !2148 (merged) on 2017-patches

Requires Brunel!1048 (merged)

@kreps @gcorti

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