Fix duplicate use of VOTE_NO_VOTES.

VOTE_NO_VOTES is defined (and used) twice in
messages.cfg and uvvote.pl - once for "no mails
to process" ("Keine Stimmen zu verarbeiten") and
once for "you didn't vote in your ballot"
("Keine Stimmen abgegeben").

Change the first use to VOTE_NO_VOTEMAILS.

Signed-off-by: Thomas Hochstein <thh@inter.net>