ref(tracing): Remove BrowserTracing logging flag default value

[lobsterkatie]

In the BrowserTracing integration, we use a flag as a way to delay logging a warning message from the constructor until we can guarantee the logger has been enabled. Until now, that flag defaulted to false, which has prevented it from being treeshaken, even in non-debug bundles in which it's not otherwise touched.

This PR makes the flag optional, and removes the default value. Since undefined is falsy there are no runtime behavior changes, but the fact that it's never used, not even to be set to a default value, allows it to be fully removed by treeshaking. Here's the difference in the non-minified, non-debug ES5 tracing bundle, before and after:

Ref: https://getsentry.atlassian.net/browse/WEB-696

[github-actions]

size-limit report

Path	Base Size (af7081c)	Current Size	Change
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.37 KB	19.37 KB	-0.02% 🔽
@sentry/browser - ES5 CDN Bundle (minified)	61.92 KB	61.92 KB	0%
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.04 KB	18.04 KB	+0.01% 🔺
@sentry/browser - ES6 CDN Bundle (minified)	55.32 KB	55.32 KB	0%
@sentry/browser - Webpack (gzipped + minified)	22.3 KB	22.3 KB	0%
@sentry/browser - Webpack (minified)	76.58 KB	76.58 KB	0%
@sentry/react - Webpack (gzipped + minified)	22.33 KB	22.33 KB	0%
@sentry/nextjs Client - Webpack (gzipped + minified)	46.56 KB	46.56 KB	-0.01% 🔽
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.25 KB	25.25 KB	-0.04% 🔽
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.65 KB	23.64 KB	-0.04% 🔽

[Lms24]

LGTM!

[AbhiPrasad]

Nice!