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_33.txt

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