ui: Remove text_top_.

After the cleanup to WearRecoveryUI, text_top_ now always equals to
((text_row_ + 1) % text_rows_).

Test: Check the recovery UI and 'View recovery logs'.
Change-Id: I69a7f377bbd990db2194f9d3efae257c323c06a8
diff --git a/screen_ui.h b/screen_ui.h
index 8231a2b..44ae692 100644
--- a/screen_ui.h
+++ b/screen_ui.h
@@ -123,7 +123,7 @@
 
   // Log text overlay, displayed when a magic key is pressed.
   char** text_;
-  size_t text_col_, text_row_, text_top_;
+  size_t text_col_, text_row_;
 
   bool show_text;
   bool show_text_ever;  // has show_text ever been true?