[ticket/12131] Remove invalid imageset var in viewtopic print view.

PHPBB3-12131
Merge pull request #1959 from prototech/ticket/12131

[ticket/12131] Remove invalid imageset var in viewtopic print view.

* prototech/ticket/12131:
  [ticket/12131] Remove invalid imageset var in viewtopic print view.
Merge remote-tracking branch 'prototech/ticket/12124' into develop