Ignore messages from all spam folders if there are many

For example, if there is both a Spam and Junk folder,
both of them should be ignored, even though only one
of them can be a ConfiguredSpamFolder.

src/config.rs

@@ -129,7 +129,6 @@ pub enum Config {
     ConfiguredInboxFolder,
     ConfiguredMvboxFolder,
     ConfiguredSentboxFolder,
-    ConfiguredSpamFolder,
     ConfiguredTimestamp,
     ConfiguredProvider,
     Configured,