Prefetch: Remove prefetch-reusable entry from VirtualTestSuites

PrefetchReusable has been enabled by default on desktop since M135:
https://crrev.com/c/6286446. We no longer need to have the separate
entry.

Note that the feature is still disabled by default on Android, but these
tests don't run on Android in the first place.

Bug: 40064891, 397882995
Change-Id: I597f710f0eb049696b7197f8bcd3ea996253cfcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6438060
Auto-Submit: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1444541}