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/codex/frame_22.txt

@@ -1,17 +0,0 @@
-                                      
-             eocdcddcdcoe             
-         ecocxoeoxoexxdxcocce         
-       odcdxecce    ecceccceeco       
-      dcxccc            ooxxxece      
-     exxoc            oeoxdxoodcx     
-    excoe           oxoxdeoe exedx    
-   xcceo           xcxecoc    xxox    
-   xxdxe           xexxee     xexcx   
-   xxoco cxddddddcceecxe eo   exdc    
-   exd ceeeo oocxoox  ecdecxoo oed    
-    eeeex cccccccce    edcceooocoe    
-     eceeeo               ecocxoc     
-      cccd cce         eococceo       
-        cdccoccxddcddodccccoc         
-           cxcxedeeeodeodce           
-