CONFIG_HEAP_MEM_POOL_SIZE is for apps, not shields

[phoddie]

Please see #99358.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[erwango]

I don't disagree on the aim of this PR, but it would need to explain why it is safe to remove this config if not other changes are made to ensure use cases remain functional.