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_19.txt

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