Fix border collapsing at the end of a table-row-group

This fixes the border-end calculation for table rows whose borders are
collapsed with rows in different rowgroups.  The border collapsing code now
uses an iterator that yields all the rows as a flat sequence, regardless of
how they are grouped in rowgroups.  It gets rid of
`TableRowGroupFlow::preliminary_collapsed_borders` which was never correct.
(It was read but never written.)

This may fix #8120 but I'm not 100% certain. (I haven't managed to reproduce
the intermittent failure locally, and my reduced test case still fails but in
a different way.)

tests/wpt/metadata-css/css21_dev/html4/border-collapse-dynamic-cell-002.htm.ini

@@ -1,3 +0,0 @@
-[border-collapse-dynamic-cell-002.htm]
-  type: reftest
-  expected: FAIL