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/default/frame_17.txt

@@ -1,17 +0,0 @@
-                                      
-                ,=+++;;~,_            
-             _;**|~~*=*|,"^,          
-            ,*\/_==`+,"|||_"\         
-           /|/_/|"    |;\~||=\        
-           |/_ ~     "/\=\//  ,       
-          `=*,/`   ,:/| /,=/|./       
-          *!;/|   ,//|_ *"||/=|       
-          -"=|!   !//||/ ,||=;*       
-          ,/*/\==+~\_|\^:\||| |       
-           |"_;__|/*\/||\!\+'+\       
-          \\\/"""****\_|*//\ \'       
-           \||_*       `/||` ;        
-            _\\!*\_   ,',/^_/         
-             , ~*+=\+`_;*:|'          
-              `+;/_,+~*_+*            
-                   `