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_4.txt

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