resolves PR change req mentioned in description

https://github.com/Semmle/ql/pull/1078/files/2affd2bef67396aa44df8ba8ab58fbec2f82ec7a#r264557539

Author: BekaValentine

javascript/ql/src/semmle/javascript/Functions.qll

@@ -184,7 +184,7 @@ class Function extends @function, Parameterized, TypeParameterized, StmtContaine
    */
 
   ConcreteControlFlowNode getAnUndefinedReturn() {
-    not (this instanceof ArrowFunctionExpr and this.getBody() instanceof Expr) and
+    not this.getBody() instanceof Expr and
     result.getContainer() = this and
     result.isAFinalNode() and
     not (result instanceof ReturnStmt and exists(result.(ReturnStmt).getExpr())) and