fix: don't filter out local instances in LCNF toMono pass (#6664)

This PR changes the toMono pass to longer filter out type class
instances, because they may actually be needed for later compilation.

src/Lean/Compiler/LCNF/ToMono.lean

@@ -150,18 +150,7 @@ where
 
 def toMono : Pass where
   name     := `toMono
-  run      := fun decls => do
-    let decls ← decls.filterM fun decl => do
-      if hasLocalInst decl.type then
-        /-
-        Declaration is a "template" for the code specialization pass.
-        So, we should delete it before going to next phase.
-        -/
-        decl.erase
-        return false
-      else
-        return true
-    decls.mapM (·.toMono)
+  run      := (·.mapM (·.toMono))
   phase    := .base
   phaseOut := .mono