Remove unneeded StringPiece header usage

Bug: 40506050
Change-Id: I15636e78efea9243ad7c13c9e91e82d683f70a35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5688848
Owners-Override: Lei Zhang <thestig@chromium.org>
Auto-Submit: Helmut Januschka <helmut@januschka.com>
Commit-Queue: Helmut Januschka <helmut@januschka.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326238}
NOKEYCHECK=True
GitOrigin-RevId: 5452f216fc8465fcb0a20970e617edcbf897d1ed