Bug 1848073 - Fix coverity UNUSED_VALUE error in OggDemuxer::SeekBisection r=padenot

`mustBackoff` is set to `false` immediately before entering a `while(true)` loop. The loop's final line is `break`, meaning that reaching a `continue` statement is necessary to loop. However, each `continue` statement is preceded by assigning `mustBackoff` to `true`. This means that `mustBackoff` will always evaluate to `false` for this loop's first iteration, and `true` for any subsequent iterations. As `mustBackoff` is only evaluated once and the evaluation happens at the beginning of the loop, assigning `mustBackoff` to `false` afterwards will have no impact on execution and the line can safely be removed.

Differential Revision: https://phabricator.services.mozilla.com/D185966

dom/media/ogg/OggDemuxer.cpp

@@ -2050,11 +2050,7 @@ nsresult OggDemuxer::SeekBisection(TrackInfo::TrackType aType,
           interval = 0;
           break;
         }
-
         backsteps = std::min(backsteps + 1, maxBackStep);
-        // We reset mustBackoff. If we still need to backoff further, it will
-        // be set to true again.
-        mustBackoff = false;
       } else {
         backsteps = 0;
       }