remove unnecessary prefix setting up msmtp.

the prefix is not correct at least for Ubuntu distro. Since doc clearly
states the dependencis. We assume msmpt is on the PATH environment.

bin/mw

@@ -101,7 +101,7 @@ mutt_profile="# vim: filetype=neomuttrc
 # muttrc file for account $title
 set realname = \"$realname\"
 set from = \"$fulladdr\"
-set sendmail = \"$prefix/bin/msmtp -a $title\"
+set sendmail = \"msmtp -a $title\"
 alias me $realname <$fulladdr>
 set folder = \"imaps://$fulladdr@$imap:$iport\"
 set imap_user = \"$login\"