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_23.txt

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