Remove the `BaseViewer._getCurrentVisiblePage` helper method
Remove the `BaseViewer._getCurrentVisiblePage` helper method

This method was originally added specifically to work-around bugs/issues related to PresentationMode in Google Chrome. Note that prior to PR 14112 we were using some CSS hacks to only show the current page in PresentationMode, and that could lead to the `getVisibleElements` function not always finding the correct elements.
However, after the changes in PR 14112 we're now using the Page-scrolling mode in PresentationMode and consequently there'll only be *a single* page visible at a time. Hence then `BaseViewer._getCurrentVisiblePage` helper method should no longer be needed, and when testing (locally) in Google Chrome everything seems to work correctly now.

Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,mozilla/pdf.js,nawawi/pdf.js,Shoobx/pdf.js