Remove strconv from disk test

[rileykarson]

#5373 made the Magician very unhappy, and goimports is even less reliable than before, failing to remove this import is one of the issues it has. I'm hesitant to revert it or #5376 as they'll force features in the GA provider to be GA which is extremely desirable. We will need to fix it up afterwards, though, likely by reducing the blasting radius of goimports.

WANT_LGTM=ANY (also feel free to merge)

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform Beta: Diff ( 9 files changed, 9 insertions(+), 3 deletions(-))

[rileykarson]

Superseded w/ https://github.com/GoogleCloudPlatform/magic-modules/pull/5392/files