refactor: Removes unused RootDiskSize field and simplify ExtraAPIInfo construction

[EspenAlbert]

Description

Removes unused RootDiskSize field and simplify ExtraAPIInfo construction

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Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

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• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
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Further comments

[EspenAlbert]

Manual run in QA: https://github.com/mongodb/terraform-provider-mongodbatlas/actions/runs/12808761623
UPDATE: Only a single OUT_OF_CAPACITY for TestAccMockableAdvancedCluster_replicasetAdvConfigUpdate (but the mocked test passed)

[lantoli]

nice refactor

[marcosuma]

it's hard to follow the why of these PRs, but it looks like the logic of this code now is changing and ZoneNameNumShards is always being set by the numShardsMapFromOldAPI method. Is that correct?

[lantoli]

it's to simplify the logic and don't rely on the plan in some scenarios as we can always rely on the Atlas response to do this. yes, numShardsMapFromOldAPI will always be used