Remove a stale transcript field assignment from the TUI (#43584)

`TranscriptState` has no `had_work_activity` field. Remove the assignment
from computer tool call completion handling to fix the resulting compile
error.

GitOrigin-RevId: 2adb15a213992e40a58dc6f217d7ae8d64c48154

codex-rs/tui/src/chatwidget/tool_lifecycle.rs

@@ -244,7 +244,6 @@ impl ChatWidget {
             );
             self.update_computer_activity(|cell| cell.complete(call, duration, result));
             self.bump_active_cell_revision();
-            self.transcript.had_work_activity = true;
             self.request_redraw();
             return;
         }