Revert "Fix #17870: XSS in adm_config_report.php"

This reverts commit b509ab380f91e84d9683dbfdb02206b88a9b86fb.

The wrong string API call was used, it should have been
string_attribute() and not string_display_line(). Thanks to
Paul Richards for pointing this out.
Fix #17870: XSS in adm_config_report.php

This is the correct fix for this issue, using string_attribute() to
escape the variable. Thanks to Paul Richards for pointing this out.