[ticket/11640] generate_text_for_display on functions_privmsgs.php

sub-task of ticket PHPBB3-11635:
find and fix all bypasses of generate_text_for_*

PHPBB3-11640
[ticket/11640] removed the unset

sub-task of ticket PHPBB3-11635:
find and fix all bypasses of generate_text_for_*

PHPBB3-11640
Merge pull request #1576 from brunoais/ticket/11640

[ticket/11640] generate_text_for_display on functions_privmsgs.php