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_36.txt

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