Remove outdated information from SubjectKeyIdentifierHashAlgorithm docs

[vcsjones]

Summary

This removes some text from the Remarks section of the SKI that are outdated.

1. SKIs are not just limited to "working with XML digital signing" so just remove it.
2. A SKI can use hash algorithms other than SHA-1, and it might hash different parts of the certificate like the subjectPublicKey or the SubjectPublicKeyInfo. What exactly is used is documented in each member of the enum, so I don't think we need to say anything about it in remarks.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @vcsjones

[learn-build-service-prod]

Learn Build status updates of commit 0c34959:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Security.Cryptography.X509Certificates/X509SubjectKeyIdentifierHashAlgorithm.xml	✅Succeeded	View

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