Remove AuthMech misspelled option (#415)

According to man page of mbsync, AuthMechs is the correct spelling of the option.

bin/mw

@@ -50,12 +50,11 @@ $starttlsoff
 "
 mbsync_profile="IMAPStore $title-remote
 Host $imap
-Port  $iport
+Port $iport
 User $login
 PassCmd \"pass mutt-wizard-$title\"
 AuthMechs LOGIN
 SSLType $ssltype
-AuthMech LOGIN
 CertificateFile $sslcert
 
 MaildirStore $title-local