[mono] Remove looking at .exe in assembly loader

[akoeplinger]

Modern .NET doesn't use .exe for managed assemblies.
Contributes to #107883

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[akoeplinger]

Hmm looks like we have a couple test failures, because of this code which changes the test assembly extension to .exe:

runtime/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/TestApp/TestApp.csproj

Lines 4 to 5 in 06f0b37

	<!-- To be consistent between net472 and others we override the SDK behavior which defaults to .dll -->
	<TargetExt>.exe</TargetExt>

This would need a bit more work so closing for now.