UI: Remove separate trash icon themeID

The removeIconSmall is now also a trash icon, so we don't need a
separate (now duplicate) themeID for it anymore.
The icon itself of course needs to stay as it's still needed.
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f