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/hash/frame_35.txt

@@ -1,17 +0,0 @@
-                                      
-              -##**###*##-            
-           #***..**#█.*.A█.**-        
-        -AA.*#*.-*█-  -█***.█.*#      
-       #AA#A#*#-            *#-*.-    
-      AA..AA* █**#            .**.#   
-     *A*A#- █.-█..*-           .**.   
-     *  .A    ** █*-.           .-**  
-     ...A      A***A█           A █A  
-     A #-    #..A*█A.**-*-*#..  ..**  
-     .#-*█  AA*█*#A.* *#----A█.A█ .█  
-      A-*#█A*-.A--  -█----*--*AA-#.   
-       █.-*A.               -A* #-    
-        **--*#A#          #*A-**█     
-          █--█.*.-A..#.*A*█-A*█       
-             █-.**--A--##*█-          
-