remove unnecessary ->header_obj calls

remove unnecessary ->header_obj calls

We used ->header_obj in the past as an optimization with
Email::MIME.  That optimization is no longer necessary
with PublicInbox::Eml.

This doesn't make any functional difference even if we were to
go back to Email::MIME.  However, it reduces the amount of code
we have and slightly reduces allocations with PublicInbox::Eml.