camerabin2: preview: remove redundant property setting on appsink

Commit 8a56a7de (camerabin2: preview: Appsink doesn't need to sync) add a line that set the "sync" property on the appsink. However, the author seems to forget that there's another property setting on appsink a few lines below.

It's very likely that the added line is required because the original line doesn't take effect (maybe because it's too late). But for whatever reason, the original line is now redundant. So, I remove it in this commit.