use go_xpost_post_filter to strip out any coauthors_plus taxonomy terms from the post to be xposted. these terms should be added by co-authors-plus on the destination blog instead of copying the author terms created on the source blog.

see https://github.com/GigaOM/legacy-pro/issues/2045
Merge pull request #20 from wluo/github-2045

strip out redundant coauthors_plus taxonomy terms in xpost data