Lib: Remove legend border

[louismaximepiton]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Designer wanted to know if we could remove the border outline around the colored squares in the legend and in the tooltip.

From

to

Motivation and context

At first the border was added because most of the colors weren't accessible against white. Now, we introduced with #106 many colors that are accessible against the background. We may need to see if we can set border on non-accessible and no border for the remaining colors.