SG15 pseudo set should not be marked as experimental. This had already been...

SG15 pseudo set should not be marked as experimental. This had already been done in 076714cdbut was unintentionally reverted, probably during a merge conflict originating from b9dd0f66.

src/species/species.F90

@@ -325,8 +325,6 @@ contains
     select case (default_pseudopotential_set_id)
     case (OPTION__PSEUDOPOTENTIALSET__NONE)
       call messages_experimental('PseudopotentialSet = none')
-    case (OPTION__PSEUDOPOTENTIALSET__SG15)
-      call messages_experimental('PseudopotentialSet = sg15')
     case (OPTION__PSEUDOPOTENTIALSET__HSCV_LDA)
       call messages_experimental('PseudopotentialSet = hscv_lda')
     case (OPTION__PSEUDOPOTENTIALSET__HSCV_PBE)