feat(sveltekit): Remove SDK initialization via dedicated files

[Lms24]

This PR removes the injectInitPlugin which means that initialization of the SDK via dedicated files is no longer supported. To initialized the SDK, the hooks.(client|server).js should be used, as already documented in the README.

(will add new stuff soon ;) )

ref #7787

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.94 KB (+0.08% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	65.44 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.49 KB (+0.08% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	57.88 KB (+0.04% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.11 KB (+0.06% 🔺)
@sentry/browser - Webpack (minified)	68.87 KB (+0.03% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.14 KB (+0.07% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	48.9 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.5 KB (+0.06% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.73 KB (+0.06% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.89 KB (-0.07% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.85 KB (-0.06% 🔽)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	63.78 KB (-0.05% 🔽)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	56.82 KB (-0.05% 🔽)