Remove `prevOnRamp` from multi onramp #2778

Job	Run time
Check Paths That Require Tests To Run	9s
Build Chainlink Image	4m 42s
Build Test Image	8m 13s
Fetch Info for Last Release	5s
CCIP Deployment with 2.12.0-ccip1.4.19-release - ccip-smoke-with-last-release	12m 35s
Upgrade Nodes with Current SHA and Run ccip-load-after-upgrade	19m 45s
	45m 29s

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