MFH: r551327

x11/gdm: Fix pkg-message

pkg-message does not contain %%PREFIX%% anymore, so a sed(1) call is not
necessary. In addition to that, passing pkg-message though fmt(1) was
breaking the UCL inside causing pkg-message to not be displayed at all.

Approved by: portmgr blanket

Approved by: ports-secteam (fluffy)