Revert of [tools] Support custom script injection for callstats.py (patchset #1 id:1 of https://codereview.chromium.org/2455623002/ )

Reason for revert:
speculative revert to address regressions on cnn.com benchmark.

Original issue's description:
> [tools] Support custom script injection for callstats.py
>
> This is a poor-man's solution to trigger page interactions.
>
> BUG=

TBR=jochen@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=

Review-Url: https://codereview.chromium.org/2464053005
Cr-Commit-Position: refs/heads/master@{#40710}