Emails: fix fatal errors when sending emails with old versions of WordPress (<4.3).
An upstream change in WordPress #WP25560 updated the version of PHPMailer bundled with WordPress, from version 5.2.4 to 5.2.7. Prior to that change, it was also impossible to use PHPMailer directly (outside of wp_mail()) due to how PHPMailer was/is bundled with WordPress. The PHPMailer change that concerns us is in PHPMailer->html2text(). A parameter to use "advanced HTML-to-text conversion" was a bool, but in the later version, it was changed to a callable parameter.
All of the above, combined with PHP's handling of truthy variables, meant we were triggering the PHPMailer bug, responsible for the fatal errors, on sites running old version of WordPress.
Fortunately, the fix is simple. We can remove our custom HTML-to-text conversion parameter as it doesn't actually matter because we always explicitly set the plain text version of the email content, after PHPMailer runs its built-in HTML-to-text conversion function, regardless of whether BuddyPress sends emails in plain text or HTML.
Fixes #6970 (2.5 branch)