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/vbars/frame_34.txt

@@ -1,17 +0,0 @@
-                                      
-               ▎▊▌▉▉▉▉▌▏▊▎            
-           ▎▉▉▉▋▍▌▏█▏▉▉▌▉▉▌▊▌▎        
-         ▎▋▋▊▏▉█▋▏▉▎   ▎▍▍▍▊█▉▉▊      
-        ▊▋▎▏▉▎▋▊▎          ▉▏▊▉▍▍     
-       ▋▋▏▍▊▋▏ ▏█ ▊          ▍▍▉▍▍    
-      ▊▋▍▏▎▋ ▍▍█▏▍▉▍          ▍▊▍▏▍   
-      ▏▉▍▏▎   █▍▎▉▏▎▍          ▏▌▏▎   
-      ▍▏▌█▋    ▏▉▎▏▋█▏         ▋ ▎▍   
-      ▊█▋▋█   ▋▍▋▋█▎▌▌▉▉▉▉▉▉▏▎▎█ ▏▍▎  
-      ▏▏▍▊▋▊▊▍█▋▋█▏▏▏▎▏▋▎▎▎▊▋▎▏▏▎▏▍   
-       ▋▎▍▊ ▏▍▏▉▍   █▉▉▎▎▎▎▍ ▊▋▊▍▏█   
-        ▋▊▍▌▌▍▎             ▋▌▋▏▋█    
-         ▍▉▎▏▉▍▌▊        ▎▉▌█▎▋▊▎     
-           ▊▍▋▏█▏▉▊▌▊▉▌▉▌▉▎▉▉▋█       
-             █▏▍▌▌▎▎▎▎▎▌▌▉▌▍          
-                   ▎▎▎