ref: Remove unneeded logic from transports

[lforst]

Previously we assumed it was necessary to return information from transport.send() to collect outcomes for client transports. The plan is to instead inject a hook/callback through the client, into the transports, that is responsible for collecting client reports within the transport itself. This makes the interface for the transports much less strict for anybody wanting to create their own transports, and it also allows us to delete a bunch of code that was used for managing the return values of transport.send(). Bundle size go ⬇️ Usability go ⬆️

This PR removes the unneeded return values from transports and anything that is now unused.

Upcoming PRs for client transports will build on these changes so keep an eye out for that. :)

Ref: https://getsentry.atlassian.net/browse/WEB-775

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.11 KB (-10.06% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	56.96 KB (-11.85% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	16.89 KB (-10.43% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	51.2 KB (-11.69% 🔽)
@sentry/browser - Webpack (gzipped + minified)	20.71 KB (-10.88% 🔽)
@sentry/browser - Webpack (minified)	71.93 KB (-11.97% 🔽)
@sentry/react - Webpack (gzipped + minified)	20.74 KB (-10.89% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.73 KB (-4.84% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	23.89 KB (-8.4% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	22.41 KB (-8.47% 🔽)

[AbhiPrasad]

❤️