Stop trying to disable searching in PresentationMode

This change was made in PR 5552, however I cannot tell why we needed to disable searching in PresentationMode. Furthermore, with the changes in PR 13908 which effectively moved where this code is invoked, searching has now (accidentally) been working in PresentationMode in e.g. the Firefox PDF Viewer for well over a year.

So, let's just enable searching unconditionally in PresentationMode to simplify the code.

web/pdf_viewer.js

@@ -1676,7 +1676,7 @@ class PDFViewer {
     return new TextHighlighter({
       eventBus,
       pageIndex,
-      findController: this.isInPresentationMode ? null : this.findController,
+      findController: this.findController,
     });
   }