Remove launched features controlling Mac camera enumeration API

Remove kUseAVCaptureDeviceDiscoverySessionDeviceMonitor and
kUseAVCaptureDeviceDiscoverySession as these have both been launched by default since M113. We'll now use the newer
AVCaptureDeviceDiscoverySession API whenever it's available.

Bug: 1427974
Change-Id: I3bbf5ddfcec0ad2c53206d563d5ba8f50758b285
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4573522


Reviewed-by: Palak Agarwal <agpalak@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Auto-Submit: Tony Herre <toprice@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1152629}