Gallery: remove CSS for mosaic mode.
Gallery: remove CSS for mosaic mode.

Since mosaic mode is replaced with thumbnail mode, we can remove these
CSS.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/1336943002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#348360}

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