flags: remove concurrent-background-loading-on-svelte

This change:
1) Removes concurrent-background-loading-on-svelte
2) Removes the backing feature offline_pages::kOfflinePagesSvelteConcurrentLoadingFeature
3) Removes all the test code that used that feature

Bug: None
Change-Id: Id8d0785a1a757c5a1b113197eeb7e2a6018d9d3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516276
Reviewed-by: Peter Williamson <petewil@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639656}
9 files changed