feat(workspaces): remove unused used field from workspaces

This commit removes the `used` field from the Workspaces database table and Zen Workspace objects.

The `used` field was originally intended to track whether a workspace was currently being used, but it turned out to be unnecessary. This change simplifies the data model and removes redundant information.

src/ZenWorkspaces.mjs

@@ -670,7 +670,6 @@ var ZenWorkspaces = new (class extends ZenMultiWindowFeature {
     let window = {
       uuid: gZenUIManager.generateUuidv4(),
       default: isDefault,
-      used: true,
       icon: icon,
       name: name,
     };

src/ZenWorkspacesStorage.mjs

@@ -18,7 +18,6 @@ var ZenWorkspacesStorage = {
           position INTEGER NOT NULL DEFAULT 0,
           created_at INTEGER NOT NULL,
           updated_at INTEGER NOT NULL,
-          used INTEGER NOT NULL DEFAULT 0
         )
       `);