Remove the "Derived Constraint" argument of TcPluginSolver, docs

docs/users_guide/extending_ghc.rst

@@ -594,7 +594,7 @@ is defined thus:
       , tcPluginStop    :: s -> TcPluginM ()
       }
 
-    type TcPluginSolver = EvBindsVar -> [Ct] -> [Ct] -> [Ct] -> TcPluginM TcPluginSolveResult
+    type TcPluginSolver = EvBindsVar -> [Ct] -> [Ct] -> TcPluginM TcPluginSolveResult
 
     type TcPluginRewriter = RewriteEnv -> [Ct] -> [Type] -> TcPluginM TcPluginRewriteResult