Remove unnecessary email headers in multisite activation emails.

BuddyPress does not set email headers for single-site activation emails,
so the same thing should occur for multisite.

Fixes issue with setting the 'MIME-Version' header twice, which could
result in the activation email being rejected in some cases.

Fixes #5186.

Props ryderlewis.  See #WP22492.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7463 cdf35c40-ae34-48e0-9cc9-0c9da1808c22