Reverts "Moving Linux linux_unopt for tree greeness (#165694)" (#165705)

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Reverts: flutter/flutter#165694
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Initiated by: jtmcdole
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Reason for reverting: bot is green (no, really, this specific bot is
green).
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Original PR Author: jtmcdole
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Reviewed By: {matanlurey}
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This change reverts the following previous change:
Unknown how many more changes are needed to make the tree go green and
close out #163012.

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Co-authored-by: auto-submit[bot] <flutter-engprod-team@google.com>

engine/src/flutter/.ci.yaml

@@ -406,7 +406,6 @@ targets:
       config_name: linux_clangd
 
   - name: Linux linux_unopt
-    bringup: true # Working through Ubuntu 24.04 issues https://github.com/flutter/flutter/issues/163012
     recipe: engine_v2/engine_v2
     timeout: 120
     properties: