Fix linting issue

src/compiler/checker.ts

@@ -33378,7 +33378,7 @@ namespace ts {
 
                 // Modifiers are never allowed on properties except for 'async' on a method declaration
                 if (prop.modifiers) {
-                    for (const mod of prop.modifiers!) { // TODO: GH#19955
+                    for (const mod of prop.modifiers) {
                         if (mod.kind !== SyntaxKind.AsyncKeyword || prop.kind !== SyntaxKind.MethodDeclaration) {
                             grammarErrorOnNode(mod, Diagnostics._0_modifier_cannot_be_used_here, getTextOfNode(mod));
                         }