Remove backward compatibility for onboarding metrics tests

[nightnei]

Proposed Changes

We replaced onboarding in this PR, but we run metrics on PR's branch and trunk branch to compare, so in that PR we added backward compatibility to metrics. And in this PR we are removing it.

Testing Instructions

All tests on this PR should be green

[github-actions[bot]]

📊 Performance Test Results

Comparing 1a82be3f6df3601b813f9aa059734351eda43548 vs trunk

site-editor

Metric	trunk	1a82be3f6df3601b813f9aa059734351eda43548	Diff	Change
load	9467.00 ms	7878.00 ms	-1589.00 ms	🟢 -16.8%

site-startup

Metric	trunk	1a82be3f6df3601b813f9aa059734351eda43548	Diff	Change
siteCreation	15206.00 ms	14355.00 ms	-851.00 ms	🟢 -5.6%
siteStartup	5948.00 ms	5979.00 ms	+31.00 ms	🔴 0.5%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change