service worker: Remove unused strings for navigation preload flag.

The ServiceWorkerNavigationPreload feature flag was removed in r494256.

Bug: 753230
Change-Id: Ie2fe09c477dbffe1bc19cc050f1e6f413b1a8358
Reviewed-on: https://chromium-review.googlesource.com/1148021
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577447}
2 files changed