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_2.txt

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