Replace EntryPointInvoker in Blazor WebAssembly with code in the runtime

[pranavkm]

.NET WASM has an API to invoke the entry point: mono_call_assembly_entry_point. We previously used it, but moved away from it since it did not support MainAsync at that point. This has since been fixed so there's no reason for us to continue using our implementation. Here's the PR that made the change: 3a93704. We would essentially want to undo the source changes, but keep the tests.

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