[clang] Remove a stale FIXME

This FIXME has already been addressed in llvm#89358

Author: hokein

clang/lib/Sema/SemaTemplate.cpp

@@ -3170,10 +3170,6 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
     Expr *RequiresClause = buildAssociatedConstraints(
         SemaRef, F, AliasTemplate, DeduceResults, IsDeducible);
 
-    // FIXME: implement the is_deducible constraint per C++
-    // [over.match.class.deduct]p3.3:
-    //    ... and a constraint that is satisfied if and only if the arguments
-    //    of A are deducible (see below) from the return type.
     auto *FPrimeTemplateParamList = TemplateParameterList::Create(
         Context, AliasTemplate->getTemplateParameters()->getTemplateLoc(),
         AliasTemplate->getTemplateParameters()->getLAngleLoc(),