Removing the flag camera_multi_client

Camera2 multi-client feature introduced in Android 25Q2 is being
backported to Android V. This changes removes the flag from the code
to enable the feature as this is already released and hence flag is not
needed.

Bug: 444512779
Test: Run SharedCameraTest
Change-Id: I9037999f004cda7450b1307fd02c156d0ab84a4b