Remove old multi-block migrations

[RomarQ]

What does it do?

This PR removes multi-block migrations applied in runtime 4000, which are no longer needed

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch rq/remove-old-migrations2

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[github-actions]

WASM runtime size check:

Compared to target branch

Moonbase runtime: 1980 KB (-8 KB) ✅

Moonbeam runtime: 2104 KB (-4 KB) ✅

Moonriver runtime: 2104 KB (-4 KB) ✅

Compared to latest release (runtime-4001)

Moonbase runtime: 1980 KB (+208 KB compared to latest release) ⚠️

Moonbeam runtime: 2104 KB (+240 KB compared to latest release) ⚠️

Moonriver runtime: 2104 KB (+240 KB compared to latest release) ⚠️

[github-actions]

Coverage Report

@@                     Coverage Diff                      @@
##           master   rq/remove-old-migrations2     +/-   ##
============================================================
  Coverage   74.62%                      74.62%   0.00%     
  Files         394                         394             
  Lines       95725                       95725             
============================================================
- Hits        71429                       71426      -3     
+ Misses      24296                       24299      +3     

Files Changed	Coverage
/runtime/common/src/migrations.rs	0.00% (-7.89%)	🔽

Coverage generated Mon Nov 24 12:02:01 UTC 2025