Remove unused undici import

[hectorhdzg]

Apparently this was added by mistake as Headers is already defined in node types, removing to fix #1386

[JacksonWeber]

I believe the reason the Node 16 tests are failing is that instanceof doesn't just use the type (which we could get from the @node/types 18.0.0 package) but requires a value, which explains why pulling the definition of the Header class from undici fixed this in Node 16 environments. We can either have an explicit dependency on undici (it's already pulled in as a dependency of the @azure/functions package) to solve the issue with yarn, or I can define a type-guard to check for the properties of the Headers type without the import.