Expunge FOLDER-SUPPORTS-MIME? and restructure INSERT-MESSAGE in

Expunge FOLDER-SUPPORTS-MIME? and restructure INSERT-MESSAGE in
imail-top.scm slightly to fix bug introduced with new MIME parser,
where non-MIME messages in folders that claim to support MIME would
cause errors by trying to walk a MIME body structure that is #F.
Folders that support MIME specially now need only to implement
different MIME-MESSAGE-BODY-STRUCTURE & WRITE-MIME-MESSAGE-BODY-PART
methods, since there is default MIME support for all folders now.