feat(sveltekit)!: Remove fetch proxy script injection and options

[Lms24]

draft as this depends on dropping SvelteKit 1.x support
closes #15007

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
20	2	18	0

View the top 2 failed tests by shortest run time

performance.test.ts captures a distributed navigation trace

Stack Traces | 2.18s run time

performance.test.ts:71:1 captures a distributed navigation trace

performance.test.ts performance events › capture a distributed navigation trace

Stack Traces | 2.27s run time

performance.test.ts:55:3 capture a distributed navigation trace

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[Lms24]

Okay so one unfortunate thing that the e2e tests surfaced: My initial fix in Sveltekit 1.26.0 missed one case where we'd load data with the stored away version of window.fetch instead of using the "current" one that our SDK patches. I opened sveltejs/kit#13315 but it means that if we move forward with this PR, we'll only fully support Sveltekit with the version from which my fix will hopefully be released with.

I think it's somewhat fine to make this tradeoff, if we document something like: The SDK supports SvelteKit 2.0.0 or greater but works best with SvelteKit 2.x.y or greater.