Remove RSVP from internal code

[NullVoxPopuli]

We don't need to use RSVP, so we can ship less bytes for consumers who also don't use it.

Unfortunately, we still need a tiny abstraction, because all these Promises have a timing behavior that I don't want to mess with right now, and that all that stuff is ran during the "async phase" of backburner

[github-actions]

📊 Size report

Tarball size — 1.2 MB → 1.2 MB

dist/dev   -0.07%↓

File	Before (Size / Brotli)	After (Size / Brotli)
./packages/@ember/application/index.js	36.4 kB / 9 kB	-1.12%↓36 kB / -0.72%↓8.9 kB
./packages/router_js/index.js	55.5 kB / 11.8 kB	-0.66%↓55.1 kB / -0.96%↓11.7 kB
./packages/shared-chunks/unrecognized-{hash}.js	17.6 kB / 4.4 kB	-1.16%↓17.4 kB / -1.96%↓4.4 kB
Total (Includes all files)	2.1 MB / 477.2 kB	-0.07%↓2 MB / -0.09%↓476.8 kB

---

dist/prod   -0.07%↓

File	Before (Size / Brotli)	After (Size / Brotli)
./packages/@ember/application/index.js	32.9 kB / 8.5 kB	-1.24%↓32.5 kB / -0.89%↓8.4 kB
./packages/router_js/index.js	55.5 kB / 11.8 kB	-0.66%↓55.1 kB / -0.87%↓11.7 kB
./packages/shared-chunks/unrecognized-{hash}.js	17.1 kB / 4.4 kB	-1.19%↓16.9 kB / -2%↓4.3 kB
Total (Includes all files)	1.9 MB / 436.8 kB	-0.07%↓1.9 MB / -0.09%↓436.4 kB

---

smoke-tests/v2-app-hello-world-template/dist   -0.1%↓

File	Before (Size / Brotli)	After (Size / Brotli)
./assets/main-{hash}.js	251.3 kB / 68.7 kB	-0.1%↓251 kB / -0.09%↓68.6 kB
Total (Includes all files)	251.6 kB / 68.8 kB	-0.1%↓251.4 kB / -0.09%↓68.8 kB

_{🤖 This report was automatically generated by wyvox/pkg-size}