Perf Tests: Simplify merger script. This script contains a lot of logic copied from Chromium scripts that we don't need. This CL removes some of it. Bug: angleproject:7671 Change-Id: I831c2620cf5d5d1325d51605122a6998fcf2d3c9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3937043 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>