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_3.txt

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