Remove a `.content-header` CSS selector.

A `.content-header` is often placed inside a `.content` div,
so this selector causes a 90% width on a container element as well as
it's child element, which is probably undesirable.

Bug: T210745
Change-Id: Ib559db66a513537127ad543cb54a119af8ebf8df

resources/skins.minerva.content.styles/tablet/common.less

@@ -112,7 +112,6 @@ A file for css that optimises the Minerva skin on larger devices.
 	/* FIXME: Generic header class needed me-thinks! */
 	.header,
 	.page-header-bar,
-	.content-header,
 	.overlay-header,
 	/* FIXME: Generic content class needed? */
 	.content,