Remove invalid ms.subservice value "subservice"

[garycentric]

No description provided.

[learn-build-service-prod]

Learn Build status updates of commit 9eea478:

✅ Validation status: passed

File	Status	Preview URL	Details
exchange/docs-conceptual/index.yml	✅Succeeded	View (exchange-ps)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 33e0600:

✅ Validation status: passed

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel