feat(GlModal): Remove variant options

BREAKING CHANGE: Remove util constants variantOptionsWithNoDefault,
and possibility to apply header, body, and footer variants.

BootstrapVue's Modal supported multiple variants:

* headerBgVariant
* headerBorderVariant
* headerTextVariant
* headerCloseVariant
* bodyBgVariant
* bodyTextVariant
* footerBgVariant
* footerBorderVariant
* footerTextVariant

These attributes were passed through the GitLab UI Component to the
BootstrapVue's original component. These variants have never been used
and are now deleted. Alongside with the exported constant in the utils
folder.