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_2.txt

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