lib-storage: Remove unnecessary mail_save_context.dest_mail==NULL checks

lib-storage: Remove unnecessary mail_save_context.dest_mail==NULL checks

It can never be NULL after the previous change: "lib-storage: Always create
mail_save_context.dest_mail".

The code removal in maildir_transaction_save_commit_pre() seemed
potentially dangerous, but I don't think such code path is possible
anymore. Also even if it is, it's probably fine since the mail_free()
is called even earlier than before (although that itself might have
been a problem).

This also removes last traces of code that made it possible to save mails to
mbox without assigning UID to the mail. The previous commit already caused
this, so this is just removing dead code.