Remove duplicate ProcessorArchitecture.MSIL branch in AssemblyNameParser (#127173)

`AssemblyNameParser.TryParseProcessorArchitecture` contained two
identical `msil` arms in its switch expression. The second was
unreachable dead code — the first arm always matched first.

Fixes #127172

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

Author: Copilot

src/libraries/Common/src/System/Reflection/AssemblyNameParser.cs

@@ -333,7 +333,6 @@ _ when attributeValue.Equals("x86", StringComparison.OrdinalIgnoreCase) => Proce
                 _ when attributeValue.Equals("ia64", StringComparison.OrdinalIgnoreCase) => ProcessorArchitecture.IA64,
                 _ when attributeValue.Equals("amd64", StringComparison.OrdinalIgnoreCase) => ProcessorArchitecture.Amd64,
                 _ when attributeValue.Equals("arm", StringComparison.OrdinalIgnoreCase) => ProcessorArchitecture.Arm,
-                _ when attributeValue.Equals("msil", StringComparison.OrdinalIgnoreCase) => ProcessorArchitecture.MSIL,
                 _ => ProcessorArchitecture.None
             };
             return result != ProcessorArchitecture.None;