Remove blankslate heading styling

[pouretrebelle]

Primer::Beta::Blankslate forces .h2 styling on the heading slot even if a non-h2 tag is passed in.

This was flagged by @rewinfrey on Slack, a lot of confusion was caused by the h4 tag having a .h2 class, and since .h2 uses !important declarations it couldn't be overridden.

The migration script to beta defaults to a h2 tag, so there are only a handful of examples that specify a different tag; in these instances the font size would be reduced with this change.

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[pouretrebelle]

Question for @ashygee - do we want to maintain the h2 sizing even if a different tag is passed in? I'm guessing that was the original intention but we'll have to add a .blankslate-heading class to Primer CSS to do that instead of using the utility (we'll have to do that as part of https://github.com/github/primer/issues/585 anyway to remove the margin utility).

[ashygee]

Question for @ashygee - do we want to maintain the h2 sizing even if a different tag is passed in? I'm guessing that was the original intention but we'll have to add a .blankslate-heading class to Primer CSS to do that instead of using the utility (we'll have to do that as part of github/primer#585 anyway to remove the margin utility).

@pouretrebelle no I don't think we should. At the time of the API audit I had made a recommendation that the default would be an h3 but that the heading size could be changed depending on the hierarchical placement of the blankslate. The original intention of the blankslate, in my mind, was to be a placeholder in larger, more spacious UI. Thus a larger h3 or h2 heading size felt appropriate. But with the discovery of the usage in more condensed UI and the fact that we do not have an alternative solution at this time, I would say that we can and should be flexible with the heading sizing.

[pouretrebelle]

Superseded by #1000