Code cleanup in excel_xml_export.php

Removed the specific logic to handle the extended text fields, which is
no longer required as the BugData API will automatically retrieve them
if needed. This is a leftover from the conversion from old, array-style
access to bug data (see 2d5e01443e2715096f2eed41b6bbca3da60d2487).

Furthermore, the call to bug_get() was redundant, since the result set
from filter_get_bug_rows() already contains BugData objects.

Fixes 0014017

Also removes a useless line of code accidentally left in there by commit
0542805d6357153fa84ad791406887ecef13cdff.