cmd/internal/obj: clean up Biobuf

This is a follow up to rev 443a32e7 which reduces some of the
duplication between methods and functions that operate on obj.Biobuf.

obj.Biobuf has Flush and Write methods as well as helpers which duplicate
those methods, consolidate on the former and remove the latter.

Also, address a final comment from CL 9525.

Change-Id: I67deaf3a163bb489a9bb21bb39524785d7a2f6c5
Reviewed-on: https://go-review.googlesource.com/9527


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>