HPCC-10838 Common up fileview2 code for writing xml

Update functions (Including: getXml, getXmlItem,
getResultCursorXml, and getResultXml) to use new IXmlWriter methods.

Remove old IStringVal specific code.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
HPCC-10838 Remove uneeded attrOnly flag in writeXmlText

The cases being checked should never happen.  The flag is confusing
and shouldn't be necessary.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5394 from afishbeck/fileviewIXmlWriterOnly

HPCC-10838 Common up fileview2 code for writing xml

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>