remove unused superchainProxyAdmin immutable variable from OPCM

[AmadiMichael]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.12%. Comparing base (4b59c05) to head (b125f59).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #17828      +/-   ##
===========================================
+ Coverage    74.93%   81.12%   +6.19%     
===========================================
  Files          174      119      -55     
  Lines        11097     6311    -4786     
===========================================
- Hits          8315     5120    -3195     
+ Misses        2636     1191    -1445     
+ Partials       146        0     -146     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	81.12% <100.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
...es/contracts-bedrock/src/L1/OPContractsManager.sol	96.60% <100.00%> (-0.01%)	⬇️

... and 55 files with indirect coverage changes

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