[cleanup] remove waitForRendering

Bug: none
Change-Id: I17446d6c7e252628f21ede90c9a4d598ac334d4b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6269194
Auto-Submit: Alex Rudenko <[email protected]>
Commit-Queue: Benedikt Meurer <[email protected]>
Reviewed-by: Benedikt Meurer <[email protected]>

Author: OrKoN