gtkselection: Drop gtk_selection_data_get_targets()

As the data isn’t set anywhere with the type “ATOM”, this doesn’t work
as expected and can be replaced by a target atom check.
4 jobs for dnd-test in 19 minutes and 47 seconds (queued for 6 seconds)
latest
Status Job ID Name Coverage
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passed #76110
fedora-x86_64

00:05:44

passed #76111
win32
msys2-mingw32

00:09:31

 
  Flatpak
passed #76112
flatpak:demo

00:05:50

passed #76113
flatpak:widget-factory

00:10:14