chore(ci): Expand cargo check to include examples

[iamjpotts]

Description:

Removes unused imports in a recently added example and adds --examples flag to cargo check command to include them in the CI check.

Related issue(s):

Follow up to https://github.com/hashgraph/hedera-sdk-rust/pull/842.

Notes for reviewer: n/a

Checklist

• [X] Documented (Code comments, README, etc.)
• [X] Tested (unit, integration, etc.)

[iamjpotts]

The cargo check --examples problems caught with this new CI step have now been fixed in https://github.com/hashgraph/hedera-sdk-rust/commit/76f5575d2236114e489ab50dd872267b42de4e1c.

Leaving PR open for a little while in case maintainers want to incorporate the expansion of cargo check into CI.