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_5.txt

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