Remove unnecessary free-var-set deletion

In TcSimplify.neededEvVars, in add_implic_seeds we were
deleting the 'givens'; but they are already deleted, so
this is a no-op.  This patch just remove the redundant
delete.

compiler/typecheck/TcEvidence.hs

@@ -399,19 +399,16 @@ data EvBindsVar
       -- Some Given, some Wanted
 
       ebv_tcvs :: IORef CoVarSet
-      -- The free coercion vars of the (rhss of) the coercion bindings
-      -- All of these are Wanted
-      --
-      -- Coercions don't actually have bindings
-      -- because we plug them in-place (via a mutable
-      -- variable); but we keep their free variables
-      -- so that we can report unused given constraints
+      -- The free Given coercion vars needed by Wanted coercions that
+      -- are solved by filling in their HoleDest in-place. Since they
+      -- don't appear in ebv_binds, we keep track of their free
+      -- variables so that we can report unused given constraints
       -- See Note [Tracking redundant constraints] in TcSimplify
     }
 
   | CoEvBindsVar {  -- See Note [Coercion evidence only]
 
-      -- See above for comments on ebv_uniq, evb_tcvs
+      -- See above for comments on ebv_uniq, ebv_tcvs
       ebv_uniq :: Unique,
       ebv_tcvs :: IORef CoVarSet
     }
@@ -834,6 +831,8 @@ evTermCoercion tm = case evTermCoercion_maybe tm of
 ********************************************************************* -}
 
 findNeededEvVars :: EvBindMap -> VarSet -> VarSet
+-- Find all the Given evidence needed by seeds,
+-- looking transitively through binds
 findNeededEvVars ev_binds seeds
   = transCloVarSet also_needs seeds
   where

compiler/typecheck/TcSMonad.hs

@@ -3340,12 +3340,12 @@ setEvBind ev_bind
 
 -- | Mark variables as used filling a coercion hole
 useVars :: CoVarSet -> TcS ()
-useVars vars
+useVars co_vars
   = do { ev_binds_var <- getTcEvBindsVar
        ; let ref = ebv_tcvs ev_binds_var
        ; wrapTcS $
          do { tcvs <- TcM.readTcRef ref
-            ; let tcvs' = tcvs `unionVarSet` vars
+            ; let tcvs' = tcvs `unionVarSet` co_vars
             ; TcM.writeTcRef ref tcvs' } }
 
 -- | Equalities only

compiler/typecheck/TcSimplify.hs

@@ -1775,8 +1775,8 @@ neededEvVars implic@(Implic { ic_given = givens
       ; return (implic { ic_need_inner = need_inner
                        , ic_need_outer = need_outer }) }
  where
-   add_implic_seeds (Implic { ic_need_outer = needs, ic_given = givens }) acc
-      = (needs `delVarSetList` givens) `unionVarSet` acc
+   add_implic_seeds (Implic { ic_need_outer = needs }) acc
+      = needs `unionVarSet` acc
 
    needed_ev_bind needed (EvBind { eb_lhs = ev_var
                                  , eb_is_given = is_given })