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_21.txt

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