[Pdg] removing Pdg from Db

src/kernel_services/plugin_entry_points/db.ml

src/kernel_services/plugin_entry_points/db.mli

src/plugins/impact/compute_impact.ml

src/plugins/impact/dune

src/plugins/impact/gui/dune

src/plugins/impact/gui/register_gui.ml

src/plugins/impact/pdg_aux.ml

src/plugins/impact/pdg_aux.mli

src/plugins/impact/reason_graph.ml

src/plugins/pdg/Pdg.ml

src/plugins/pdg/Pdg.mli

src/plugins/pdg/annot.ml

src/plugins/pdg/api.ml

src/plugins/pdg/api.mli

src/plugins/pdg/marks.ml

src/plugins/pdg/pdg_tbl.ml

src/plugins/pdg/pdg_tbl.mli

src/plugins/pdg/register.ml

src/plugins/postdominators/print.ml

src/plugins/scope/defs.ml