Before organizing the dark mode test, we had same tests with different
names different in folders like filter-all, filter-none and grayscale.
This patch removes duplicate web tests for gradient and image.

Bug: 1116914
Change-Id: If36468417c84ed122e0ce612569f7b2f8e8c30e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2409879Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Prashant Nevase <prashant.n@samsung.com>
Cr-Commit-Position: refs/heads/master@{#806892}