Remove unused include.

Missed removing this in
https://chromium-review.googlesource.com/c/chromium/src/+/2261216

TBR=alexmos@chromium.org

Bug: 1085275
Change-Id: Idb1fe791f8e041f89654500ba768c6f1b23a4ef0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305032Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789633}

content/browser/site_instance_impl.h

@@ -5,8 +5,6 @@
 #ifndef CONTENT_BROWSER_SITE_INSTANCE_IMPL_H_
 #define CONTENT_BROWSER_SITE_INSTANCE_IMPL_H_
 
-#include <functional>
-
 #include <stddef.h>
 #include <stdint.h>