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_32.txt

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