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/dots/frame_25.txt

@@ -1,17 +0,0 @@
-                                      
-             ○●○●◉●●●●○               
-           ● ●●●·○◉○·●··●             
-          ◉·◉◉●● ◉·●○○○···○           
-         ·●●· ◉  ○·  ○·●●○◉           
-        ●○○○●◉●○◉◉○·  ·◉◉○◉●          
-        ○●···○●●◉○·◉ ◉·●·◉◉·●         
-        ·◉·●· ·○○◉○○◉·○◉○ ·○·         
-        ···◉·◉  ·○○○◉·○○·○··○         
-       ○●○· ·○○○○○○●··○○·●●·○         
-        ◉◉ ·◉·●·○··◉●○·○●○◉○·         
-        ○·●◉○··     ●●◉·○·◉··         
-         ·○ ·◉●◉○◉●●◉○◉●◉◉·◉          
-          ○·○·○○○●◉◉●○◉○··◉           
-           ○◉◉○○○●○·●● ●·●            
-             ·● ●○●●○●·●              
-