Revert "ignore application/pgp-signature parts for display"

This reverts commit e8c591e6be.

Since crypto information is stored in annotations now rather than the
mime tree itself, this doesn't need to be handled any longer

k9mail-library/src/main/java/com/fsck/k9/mail/internet/MessageExtractor.java

@@ -166,8 +166,6 @@ public class MessageExtractor {
                 Html html = new Html(part);
                 viewables.add(html);
             }
-        } else if (part.getMimeType().equalsIgnoreCase("application/pgp-signature")) {
-            // ignore this type explicitly
         } else {
             // Everything else is treated as attachment.
             attachments.add(part);