LaTeX/Beamer templates: Remove conditionalization around date.

This restores the old behavior, where no date was printed
if no date was specified.  If the `\date` command is removed
entirely, standard document classes will print today's date.
Closes #1321.

Partially reverts fc5229df16.
See #61.

default.beamer

@@ -130,9 +130,7 @@ $endif$
 $if(author)$
 \author{$for(author)$$author$$sep$ \and $endfor$}
 $endif$
-$if(date)$
 \date{$date$}
-$endif$
 
 \begin{document}
 $if(title)$

default.latex

@@ -137,9 +137,7 @@ $endif$
 $if(author)$
 \author{$for(author)$$author$$sep$ \and $endfor$}
 $endif$
-$if(date)$
 \date{$date$}
-$endif$
 $for(header-includes)$
 $header-includes$
 $endfor$