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_34.txt

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