Consider removing direct dependency on npx

[ballPointPenguin]

The distributed npx library is deprecated and now is handled within the npm distribution.
https://github.com/npm/npx#readme

This means that the npx library being required here will be stuck at v10.2, which in turn requires npm 5.x, which (I think) is sometimes causing problems with ember-cli-update because it uses an older version of graceful-fs.

I realize this is a long cause-and-effect chain of my insinuation. I am experiencing cb.apply is not a function when trying to run ember-cli-update, and so far this is the best guess that I have.

[ballPointPenguin]

Related to ember-cli/ember-cli-update#1105

[kellyselden]

I think enough time has progressed, and we can assume npx is already on the machine. I think we can remove it.

[aberres]

@kellyselden Out of curiosity, I ended up here when looking into why we install an old deprecated tar. Any plans to drop the dependency?

[kellyselden]

I'm open to the change, but I'm not planning on doing the work myself.