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/default/frame_31.txt

@@ -1,17 +0,0 @@
-                                      
-                _.:*+*+=,             
-              _+*;+,_"+\'*,           
-             ,|\//,=_`."|_*\          
-            ,~/+__*;_,\\|\~|\         
-            ^||||+-*\_,"\|/__         
-           ;|^`~_|'"\;*,./|,"|        
-           /|| |^*|\.=/;*|*|/|        
-           \\, |~"|\ |^""|~\.|        
-           |\,_|'^~|~/+~~|_  |        
-           "||~//||___\_|\|| *        
-           ^*_/+|, /***`/|'~_!        
-            |||\\\,     _=* |         
-             ;|*=_!,  . |*`/`         
-              :|\|/_|`,|",|`          
-               '"=~_+*/.;*            
-                   ````