fix: properly purge server load cache

[coyotte508]

fixes #8272

When going from a page with server load to a page without server load, the cache wasn't purged properly

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You can check out a repo with the bug here: https://github.com/coyotte508/sveltekit-load-bug/

[Rich-Harris]

Thank you! Any chance you could add a test for this? It could go in this section:

kit/packages/kit/test/apps/basics/test/client.test.js

Line 42 in d1f7655

test.describe('Load', () => {

[dummdidumm]

Thank you!