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_23.txt

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