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_24.txt

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