Remove legacy bootstrap stylesheets

[joshuarrrr]

As part of our audit of typographic styles we discovered that, when OpenSearch Dashboards starts up, it bundles together 3 stylesheets:

1. The selected OUI theme CSS
2. The KUI theme CSS
3. A "legacy" stylesheet that consists mostly of Bootstrap resets

To avoid a deep and time-consuming investigation into the expectedly rare cases where these legacy styles actually have a visible effect, we'll simply remove them from our feature branch and test endpoint and visually check for inconsistencies.