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 18ad379c92db070115b8921bd51024b5bb5dd230).

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
b5450f3d7eedf5669affe77865560cbf053bbead.