lang/rust: unbreak FFI for returning struct on i386

As this fixes Stylo on i386, remove the workaround in www/firefox.

PR: 223047
Suggested by: Sebastien Marie <semarie@openbsd.org>
Approved by: rust (dumbbell)
MFH: 2017Q4 (required by Firefox 57 scheduled on 2017-11-14)