...i.e., "const X = N;" declarations outside of constant groups.  This appears
to be unused in practice, and removing it will allow to simplify future code
related to UNO type information.

This removes the definition of "constant-decl", and the branch mentioning
"constant-decl" from the definition of "declaration" at
<http://wiki.openoffice.org/w/index.php?title=Documentation/DevGuide/AppendixD/UNOIDL_Syntax_Specification&oldid=137490>.

Change-Id: I218626f02ebd46ee438bf5453ca1510c9ed8e5f0