test(rivetkit): remove stale http stream coverage

[eersnington]

Description

• cargo test -p rivetkit-napi does not compile on main. The crate's test module still has cancelling_http_request_body_drops_core_receiver, which imports HttpRequestBodyStream from rivetkit_core and from the NAPI wrapper.
• Both types were removed when SSE streaming was cleaned up (e534b6e) and the dangling core re-export was dropped (d44805d). The test outlived what it tested.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

# Before, on main:

$ cargo test -p rivetkit-napi --no-run
error[E0432]: unresolved import `rivetkit_core::HttpRequestBodyStream`
error[E0432]: unresolved import `super::HttpRequestBodyStream`
error: could not compile `rivetkit-napi` (lib test) due to 2 previous errors

# After:

$ cargo test -p rivetkit-napi --no-run
    Finished `test` profile [unoptimized + debuginfo] target(s) in 3.73s

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[eersnington]

closing since a fix is merged 8b57037