[scenic] Remove dead view_manager ref

[arbreng]

Description

Fuchsia doesn't use view_manager anymore, so remove this from the CMX template

Related Issues

N/A

Tests

N/A, no behavior change

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[zanderso]

lgtm

For Flutter PRs the practice is to wait for an lgtm before merging.

[arbreng]

Thanks for the review; I'msorry about that! The change seemed so safe that I figured I could just merge it this time.