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_7.txt

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