Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/tui/frames/slug/frame_2.txt

@@ -1,17 +0,0 @@
-                                      
-             d-dcotooottd             
-         dtt5pcteexoxeodpeptd         
-       tepeoppxpee  egpop5eecet       
-     de5d5ppttd           -toe5et     
-     tdg5pdeodood           dteoet    
-    p5tge  epot5ot           ooepe    
-   teppe    d5ecedet          5gege   
-   eg oe     tepeecp          ep5-e   
-   pggoe    cedddeg-xtttttttttedexp   
-    dope  5eep 5p eoodddd--ddet5geg   
-    ooo p po--ep   egpppppppgetpee    
-     pod-5t e              ttc5tp     
-       -oddett          todtgdeg      
-        exdcddgptccocc-opedeep        
-           eptptxxddddxc5pg           
-