Simplify tf.ENV.registerBackend (remove setTensorTracker param)

[dsmilkov]

There is no need for custom backends to provide their own tensor trackers since the tracker is the engine (a global).

Tested that double tfjs import works:

<script src="tf-core.min.js"></script>
<script src="tf-core.min.js"></script>
<script>
	tf.square(3).print();
</script>

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This change is 

[nsthorat]

Reviewed 4 of 4 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @nsthorat and @pyu10055)

[pyu10055]

Thanks!

Reviewed 4 of 4 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @pyu10055)