ref(vue): Reduce bundle size for starting application render span

[Lms24]

Just a smal bundle size reduction PR for the Vue SDK: I discovered while working on #14251 that we can simplify the check here and just use the onlyIfParent flag instead of querying the active span. This should behave identically.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	22.77 KB	added	added
@sentry/browser - with treeshaking flags	21.53 KB	added	added
@sentry/browser (incl. Tracing)	35.26 KB	added	added
@sentry/browser (incl. Tracing, Replay)	71.98 KB	added	added
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	62.35 KB	added	added
@sentry/browser (incl. Tracing, Replay with Canvas)	76.31 KB	added	added
@sentry/browser (incl. Tracing, Replay, Feedback)	89.14 KB	added	added
@sentry/browser (incl. Feedback)	39.93 KB	added	added
@sentry/browser (incl. sendFeedback)	27.42 KB	added	added
@sentry/browser (incl. FeedbackAsync)	32.23 KB	added	added
@sentry/react	25.52 KB	added	added
@sentry/react (incl. Tracing)	38.22 KB	added	added
@sentry/vue	26.91 KB	added	added
@sentry/vue (incl. Tracing)	37.09 KB	added	added
@sentry/svelte	22.91 KB	added	added
CDN Bundle	24.13 KB	added	added
CDN Bundle (incl. Tracing)	37.04 KB	added	added
CDN Bundle (incl. Tracing, Replay)	71.7 KB	added	added
CDN Bundle (incl. Tracing, Replay, Feedback)	77.05 KB	added	added
CDN Bundle - uncompressed	70.73 KB	added	added
CDN Bundle (incl. Tracing) - uncompressed	109.9 KB	added	added
CDN Bundle (incl. Tracing, Replay) - uncompressed	222.42 KB	added	added
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	235.64 KB	added	added
@sentry/nextjs (client)	38.33 KB	added	added
@sentry/sveltekit (client)	35.84 KB	added	added
@sentry/node	134.28 KB	added	added
@sentry/node - without tracing	96.46 KB	added	added
@sentry/aws-serverless	106.72 KB	added	added