Drop redundant styles from .grab-to-pan-grab:active

`.grab-to-pan-grab:active` is `#viewerContainer` when the mouse is
pressed down.  It is supposed to have a `cursor: grabbing` appearance
immediately on mousedown,

`.grab-to-pan-grabbing` is the overlay that is supposed to cover
everything, and also has the `cursor: grabbing` appearance. The "cover
everything" result is achieved through `position:fixed`, `inset:0`, etc.

The block with these CSS properties for "cover everything" is currently
shared by `.grab-to-pan-grab:active` and `.grab-to-pan-grabbing`, but
only "cursor" need to be shared. The original JS and CSS code at
https://github.com/Rob--W/grab-to-pan.js shows that these were supposed
to be associated with the overlay only.

The PR that added this to PDF.js also shows that the "cover everything"
CSS properties were supposed to be limited to the overlay only:
https://github.com/mozilla/pdf.js/pull/4209#discussion-diff-9285917

But the final version of the PR mistakenly merged them together.
This patch rectifies that mistake.

web/viewer.css

@@ -1368,6 +1368,8 @@ dialog :link {
 .grab-to-pan-grab:active,
 .grab-to-pan-grabbing {
   cursor: grabbing !important;
+}
+.grab-to-pan-grabbing {
   position: fixed;
   background: rgba(0, 0, 0, 0);
   display: block;