compositor: Do not repick after effects finish

This was a temporary fix until there was a better crossing event delivery mechanism that accounted for actor changes beneath the pointer. We nowadays have that, and don't seem to need this extra kick to get crossing events triggered (and cursor changes, etc) when windows appear or disappear under the pointer.

This commit is effectively a revert of commit a64dba4d.

Closes: gnome-shell#6808 (closed)