Change rxjs-report-usage to devDependency

[dobesv]

It seems like rxjs-report-usage is a command-line tool intended to be run be a developer if they so choose.

Having it as a runtime dependency adds a bunch of unnecessary code to deployed code.

The tool depends on babel which is a pretty big package, so it's more than just the little script itself, but also a decent chunk of babel

[cartant]

I've removed it entirely in 10.6.2.