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_18.txt

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