[Markdown] Replace custom styling with OUI.

[curq]

According to the OUI usage audit of vis_type_markdown plugin. There are custom stylings present and better OUI compliance is needed. Ideally all the styles should be replaced with OUI

OpenSearch-Dashboards/src/plugins/vis_type_markdown/public/markdown_vis.scss

Lines 8 to 22 in fe1f1d1

	.mkdVis {
	padding: $euiSizeS;
	width: 100%;
	}
	.visEditor--markdown {
	.visEditorSidebar__config > *,
	.visEditor--markdown__textarea {
	flex-grow: 1;
	}
	.mkdEditor {
	height: 100%;
	}
	}