fix(GlChartLegend): remove tooltips from labels of tabular legends

What does this MR do?

Removes the tooltips from the tabular chart legend labels, as it was discovered during a UX review that the cases where they encumber the UI far outweigh the cases where they help (i.e. when the legend labels are too long to fit).

Relevant conversation here: !1385 (comment 345779374)

Tooltip example

Related issue

#812 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.