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_4.txt

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