Removed outdated comment from lib/ui/canvas.h (#43112)

This no longer applies.
https://dart.googlesource.com/external/github.com/flutter/engine/+/cfc1f3afa435616bebdf9e35d58ef4dcd5db2ab2
1 file changed
tree: 97c3b420e717e556cb8bafde3e788813de439671
  1. commits.json