clutter: Remove drag and drop actions

We aren't using those actions in the shell or anywhere in Mutter, our
DnD support is implemented on the shell side.

GNOME/mutter!789
5 jobs for !789 with remove-drag-drop-action in 8 minutes and 47 seconds
latest detached
Status Job ID Name Coverage
  Review
passed #428322
check-commit-log

00:00:21

 
  Build
passed #428323
build-mutter

00:05:35

passed #428324
build-without-native-backend

00:02:02

 
  Test
passed #428326
can-build-gnome-shell

00:01:15

passed #428325
test-mutter

00:02:50