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_1.txt

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