Drop the “nested dfn or span in heading” error

This change drops an error that Wattsi emits for the case where a
heading has an empty span or dfn (that is, with no text content),
followed by another span or dfn in the same heading.

The current code appears to have been (for some reason that’s not
obvious now) very intentionally written to report an error for exactly
that case — but not for the case where of a heading with *non-empty*
span or dfn followed by another span or dfn in the same heading.

Without this change, Wattsi reports an error for a reasonable markup case
like the one in whatwg/html#5855; that is, this:

<h5 id="rel-alternate"><span id="link-type-alternate"></span>Link type "<dfn data-export=""
data-dfn-for="link/rel,a/rel,area/rel" data-dfn-type="attr-value"><code
data-x="rel-alternate">alternate</code></dfn>"</h5>

It’s not at all obvious why we’d want a case like that to be an error.

src/wattsi.pas

@@ -1286,8 +1286,6 @@    TCrossReferences = record
                      begin
                         if ((CandidateChild is TElement) and ((TElement(CandidateChild).IsIdentity(nsHTML, eDFN)) or (TElement(CandidateChild).IsIdentity(nsHTML, eSpan)))) then
                         begin
-                           if (InSkippedNode) then
-                              Fail('Nested <dfn> or <span> elements in heading ' + LastSeenHeadingID);
                            InSkippedNode := True;
                            if (CandidateChild.HasChildNodes()) then
                            begin