MM-52451: always show mark all threads as read button (#23100)

Enables the "Mark All Threads as Read" button even when you don't have
unreads.
This is done as a temporary action to mitigate the phantom unread
mentions pending issue.

webapp/channels/src/components/threading/global_threads/thread_list/__snapshots__/thread_list.test.tsx.snap

@@ -52,7 +52,6 @@ exports[`components/threading/global_threads/thread_list should match snapshot 1
         >
           <Memo(Button)
             className="Button___large Button___icon"
-            disabled={false}
             id="threads-list__mark-all-as-read"
             marginTop={true}
             onClick={[Function]}

webapp/channels/src/components/threading/global_threads/thread_list/thread_list.tsx

@@ -233,7 +233,6 @@ const ThreadList = ({
                         >
                             <Button
                                 id={'threads-list__mark-all-as-read'}
-                                disabled={!someUnread}
                                 className={'Button___large Button___icon'}
                                 onClick={handleOpenMarkAllAsReadModal}
                                 marginTop={true}