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_34.txt

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