fix(GlModal): Hide close button if modal-header slot used

What does this MR do?

Hide close button when using custom modal header slot. Review app

Required for gitlab!82687 (closed) - we are adding a button next to the close button so need to customise the look of the button.

Previously we always showed the close button with no way to remove it, which meant the #modal-header and #modal-title slots did basically the same thing. Following this change using the #modal-header slot will not show the close button.

Match the behaviour of b-modal - https://bootstrap-vue.org/docs/components/modal

Also, if you use the modal-header slot, the default header X close button will not be present, nor can you use the modal-title slot.

before	after

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