net-p2p/transmission-gtk: Fix .desktop file when NLS disabled.

When the NLS option is disabled the installed transmission-gtk.desktop file
contains fields that start with and underscore, like _Name instead of Name.
When the NLS option is enabled these are replaced by translations. The attached
patch adds a simple sed command that removes the underscores when NLS is
disabled.

PR: 240125
Submitted by: tijl
Approved by: tcberner (mentor)