…xt refresh (electron#51329)

fix: remove early capturer_.reset() that causes null deref on next refresh

Another followup to dad4ab6: remove the `capturer_.reset()` that
`desktop_media_list.patch` was adding `Worker::RefreshNextThumbnail()`.
Since we switched from the one-shot Update() model to the continuous
StartUpdating() model, resetting `capturer_` isn't necessary and is
now dangerous: ScheduleNextRefresh() posts a delayed Worker::Refresh()
that dereferences capturer_, causing a nullptr crash.

Under CI load, the NativeDesktopMediaList can survive long enough
for the next 1-second refresh cycle to fire before FinalizeList()
destroys it. The crash can manifest as either a SIGSEGV or a
DCHECK(can_refresh()) failure, which is extra fun because dad4ab6
was fixing a similar DCHECK crash in the first place.

Sample crash:

```
[6690:0426/173732.876803:FATAL:chrome/browser/media/webrtc/native_desktop_media_list.cc:934] DCHECK failed: can_refresh().0x00000001337aa7f3 NativeDesktopMediaList::RefreshForVizFrameSinkWindows(...) + 131
```