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_1.txt

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