As the documentation for next_post_link
states, the link parameter is where the actual text of the %link placeholder goes. The format parameter is the whole text, where %link is unpacked from the second link parameter. Solution:
next_post_link( '%link', '%title →' );
Expected output: <a href="https://wordpress.stackexchange.com/questions/45044/...">Title →</a>
Something like this, on the other hand:
next_post_link( 'Over to %link now!', '-- %title --' );
…will result in: Over to <a href="https://wordpress.stackexchange.com/questions/45044/...">--- Title ---</a> now!
See how these are unpacked?