Remove useless includes from wtf_string.h

BUG=242216

Change-Id: Iea525e5c1091aa85193b23edbedead8bf997ab05
Reviewed-on: https://chromium-review.googlesource.com/c/1286907
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600520}
1 file changed