refactor(charts): Remove unused ToolboxMixin

What does this MR do?

During investigation regarding #1902 (closed) I realise that ToolboxMixin is never used in gitlab-ui or gitlab projects.

How did I arrive at this conclusion?

• without props showToolbox set to true the mixin has no effect (https://gitlab.com/gitlab-org/gitlab-ui/-/blob/1e555b5f762a35b83c619a0744ff00aae2dee87a/src/components/mixins/toolbox_mixin.js#L36-43)
• the only occurrence found of showToolbox or show-toolbox in gitlab-ui and gitlab projects is :show-toolbox="false" in cycle_analytics/components/duration_chart and was removed by gitlab!104378 (merged)

Blocker: I open a MR to remove the only reference of showToolbox in gitlab project: gitlab!104378 (merged)

This MR remove :

• src/components/mixins/toolbox_mixin.js
• all ToolboxMixin references in charts components and charts Storybook stories
• getToolboxConfig in utils/charts/config.js

There is no visible changes.

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
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.