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_22.txt

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