[fuchsia] Update README.chromium for fuchsia-sdk Also remove the copy of the LICENSE file and an obsolete SDK path in third_party/.gitignore. Change-Id: Id67cb2bfec128572f23f8dcdccc08b54a2601338 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427964 Reviewed-by: Wez <[email protected]> Reviewed-by: Sergey Ulanov <[email protected]> Commit-Queue: David Dorwin <[email protected]> Cr-Commit-Position: refs/heads/master@{#812846} GitOrigin-RevId: 5b85e230f8028f804df4d434afc13f2f64929e3f