(core) Removing the new menu flag

Summary: Enabling the `GRIST_NEW_COLUMN_MENU` flag by default and removing it.

Test Plan: Existing

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4098

app/common/gristUrls.ts

@@ -701,9 +701,6 @@ export interface GristLoadConfig {
 
   permittedCustomWidgets?: IAttachedCustomWidget[];
 
-  // Feature flag for the new column menu.
-  gristNewColumnMenu?: boolean;
-
   // Used to determine which disclosure links should be provided to user of
   // formula assistance.
   assistantService?: 'OpenAI' | undefined;