Depsgraph: remove unused no-op nodes after building

This is the companion of D7031. That patch adds a new DIMENSIONS node to
the depsgraph for each object that has geometry. However, this node is
only necessary when there are drivers using an object's dimensions as
variable. Since this is rare, it's easiest to remove these nodes after
they turn out to be unnecessary. This is what (almost) happens in this
patch.

Removing nodes from the depsgraph is hard, and there are no functions to
do this yet. Instead, this patch recursively removes all the incoming
relations from unused no-op nodes (i.e. no-op operation nodes without
outgoing connections). Actually removing the nodes will be left as a
future improvement.

I've tested this on a Spring file [1]. Here are there results of blender
--debug-depsgraph-time spring_02_055_A.eevee.blend and letting it run
for a while to stabilise the reported FPS:

    master: 11.7 FPS
    Just D7031: 11.7 FPS
    Just D7033: 11.8 FPS
    Both D7031 + D7033: 12.3 FPS

[1] https://cloud.blender.org/p/spring/5d30a1076249366fa1939cf1

Differential Revision: https://developer.blender.org/D7033

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