Per previous convention (see
ace397e9), drop SQL:2008 and only keep
the latest two standards and SQL-92.

Note: SQL:2016-2 lists a large number of non-reserved keywords that
are really just information_schema column names related to new
features.  Those kinds of thing have not previously been listed as
keywords, and this was apparently done here by mistake, since these
keywords have been removed again in post-2016 working drafts.  So in
order to avoid bloating the keywords table unnecessarily, I have
omitted these erroneous keywords here.