Delete the `blocklyTreeRowContentContainer` div

Remove the `rowContents_` div from the [toolbox category](https://github.com/google/blockly/blob/f8025a1bdd12025ecb8830159509148ff27ad3c8/core/toolbox/category.ts). Afaict it doesn't actually do anything because its its only styling and doesn't have any styling.

This is still a breaking change though because folks may be relying on this for styling or in downstream category classes.