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_20.txt

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