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_32.txt

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