Remove the useless PresentationModeState.CHANGING-case in PDFCursorTools (PR 12788 follow-up)

For reasons that I now can't for the life of me understand, I included handling of the `PresentationModeState.CHANGING`-case despite it not actually doing anything.

web/pdf_cursor_tools.js

@@ -129,8 +129,6 @@ class PDFCursorTools {
 
     this.eventBus._on("presentationmodechanged", evt => {
       switch (evt.state) {
-        case PresentationModeState.CHANGING:
-          break;
         case PresentationModeState.FULLSCREEN: {
           const previouslyActive = this.active;