Don't include any gentext in XREFs

[sandersk]

If no gent ext is specified in AsciiDoc for an XREF, can we leave the text nodes of HTMLBook XREF elements empty when converting to HTMLBook. In other words, can we convert this:

See <<the_id>>

to:

See <a data-type="xref" href="#the_id"/>

Instead of what we're doing now, which appears to be:

See <a data-type="xref" href="#the_id">[#the_id]</a>

Downstream, the HTMLBook XSL stylesheets will handle adding the appropriate gentext if an XREF has no text node, so I think it makes most sense to defer to having it handled there, so we can avoid replicating existing functionality.

If AsciiDoc XREFs do have gentext specified, we should preserve that, though. For example, if you've got:

See <<the_id, Awesome Chapter>>

It should be converted to:

See <a data-type="xref" href="#the_id">Awesome Chapter</a>

Let me know if you have any questions about this.

Thanks,
Sanders