applets/taskmanager: don't show Unpin menu item while app is launching

We already filter out the Pin menu item for launching apps, but doing so
for the Unpin menu item was missed. Showing this menu item makes even
less sense for launching apps, so it appears to have been a simple
omission rather than an intentional design choice.

BUG: 468668
FIXED-IN: 5.27.5