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_20.txt

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