Remove the merge command from roll_aosp.sh
Remove the merge command from roll_aosp.sh

There are two issues doing the merge in the script:
 - This command makes the merge specific to an AOSP branch
 - It uses the version of roll_aosp that is from the previous roll
   instead of the ToT version.

BUG=b:150892231

Change-Id: I579484092ca3d8c5e0e2d4d4bd1272eaeccc9da3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2152667
Reviewed-by: Tim Van Patten <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>
Commit-Queue: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle