Provide valid C# example and remove C++/CLI function definition.

[AaronRobinsonMSFT]

Fixes #1120

/cc @dotnet/docs @elinor-fung @jkoritzinsky

[opbld31]

Docs Build status updates of commit 137accf:

✅ Validation status: passed

File	Status	Preview URL	Details
samples/snippets/csharp/VS_Snippets_CLR_System/system.runtime.interopservices.icustommarshaler/cs/source.cs	✅Succeeded	View
xml/System.Runtime.InteropServices/ICustomMarshaler.xml	✅Succeeded	View

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[AaronRobinsonMSFT]

@gewarren PTAL. Thanks!

[gewarren]

Looks good, just tweaked the intro to the example.

[opbld31]

Docs Build status updates of commit 1370afa:

🕙 Pending: waiting for processors (14 builds ahead of you)

⚠️ Docs Build is busy, currently there are 14 builds ahead of this one, for more information you can view the Build queue graph on the Docs Portal.

[opbld33]

Docs Build status updates of commit 1370afa:

✅ Validation status: passed

File	Status	Preview URL	Details
samples/snippets/csharp/VS_Snippets_CLR_System/system.runtime.interopservices.icustommarshaler/cs/source.cs	✅Succeeded	View
xml/System.Runtime.InteropServices/ICustomMarshaler.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching in the Docs contributor and Admin Guide
• See the frequently asked questions
• Post your question in the Docs support channel