update configuration to deal with pgun regardless of detector configuration
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.
Requires Brunel!1046 (merged)
Can be cherry-picked to 2018-patches. Patch for 2016 is !2149 (merged).