Remove now-unused InstallPlan.preexisting

We only ever switch Configured to Installed. The PreExisting state only
comes from the original solver plan, which only uses installed packages
from the global db.

cabal-install/Distribution/Client/InstallPlan.hs

@@ -33,7 +33,6 @@ module Distribution.Client.InstallPlan (
   fromSolverInstallPlan,
   configureInstallPlan,
   remove,
-  preexisting,
   installed,
   lookup,
   directDeps,
@@ -291,26 +290,6 @@ remove shouldRemove plan =
     newIndex = Graph.fromList $
                  filter (not . shouldRemove) (toList plan)
 
--- | Replace a ready package with a pre-existing one. The pre-existing one
--- must have exactly the same dependencies as the source one was configured
--- with.
---
-preexisting :: (IsUnit ipkg,
-                IsUnit srcpkg)
-            => UnitId
-            -> ipkg
-            -> GenericInstallPlan ipkg srcpkg
-            -> GenericInstallPlan ipkg srcpkg
-preexisting pkgid ipkg plan = plan'
-  where
-    plan' = plan {
-      planIndex   = Graph.insert (PreExisting ipkg)
-                    -- ...but be sure to use the *old* IPID for the lookup for
-                    -- the preexisting record
-                  . Graph.deleteKey pkgid
-                  $ planIndex plan
-    }
-
 -- | Change a package in a 'Configured' state to an 'Installed' state.
 --
 -- To preserve invariants, the package must have all of its dependencies