Bug 10720: fix issue that prevented use of HTML tags in overdue notices

Substitution term in overdue notices shouldn't be removed if they don't match.
It prevents use of HTML in notices to use with CSS.

Test plan :
- define an overdue notice containing some valid substitution terms + some valid html tags (<strong> or <div class="my class" to use with a css
- find a borrower with overdues that trigger this notice
- run overdue_notices.pl -html <directory>

Without patch, your html tags are missing in your html file
With the patch, they are present and can be used with a css to generate a formatted pdf file.

If you add some invalid substitution term, with the patch, they are still in the final letter, but the warn in the log helps you to correct your notice template.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>