Remove `MockClock()` (#18992) #1641

Job	Run time
Build and push image for linux/amd64	2m 52s
Build and push image for linux/arm64	2m 49s
Push merged images to ghcr.io/element-hq/synapse	48s
Push merged images to docker.io/matrixdotorg/synapse	40s
	7m 9s

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