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_25.txt

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