[Cocoa] Remove SPI declarations for AVSampleBufferRenderSynchronizer
<rdar://problem/112164268>
Pull request: https://github.com/WebKit/WebKit/pull/16880
Committed 267080@main (cf8be6182eeb): <https://commits.webkit.org/267080@main> Reviewed commits have been landed. Closing PR #16880 and removing active labels.