[soundwave] Cleanup .gitignore
[soundwave] Cleanup .gitignore

Soundwave no longer stores its own credentials. This file should not
exist any more.

TBR=benjhayden@chromium.org

Bug: chromium:903026
Change-Id: I5af910f99ef157440149508879cecc103930f167
Reviewed-on: https://chromium-review.googlesource.com/c/1349362
Reviewed-by: Juan Antonio Navarro Pérez <1649bb5e6b9eafe026580e338065dc64ba6b80b2@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <1649bb5e6b9eafe026580e338065dc64ba6b80b2@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult