This removes all internal uses of `jQuery.support.boxModel`. jQuery has never run unit tests with Quirks Mode and has not even feigned support for several years, so these remnants weren't doing much except giving false hope.

For now, `jQuery.support.boxModel` continues to have a value indicating whether the W3C box model is *generally* in use, but be aware that this is easily overridden on an element-by-element basis by the `box-model` CSS property. So don't trust this value.