Remove manual Headers iterator registration

[itisrohit]

This Pull Request is a follow-up to #4697, which closed #4688.

It changes the following:

• Removes the remaining manual Headers.prototype[Symbol.iterator] registration from fetch runtime setup.
• Uses the existing #[boa(symbol = "iterator")] class method as the single implementation path.
• Updates the invalid receiver regression test to assert the TypeError contract instead of the old manual error message.

Tests run:

• cargo fmt --check
• git diff --check
• cargo test -p boa_runtime headers
• cargo test -p boa_engine recursion_in
• cargo test