Make leading-none a static utility

[adamwathan]

This PR removes the --leading-none variable from the default theme in favor of making leading-none a static utility, since it doesn't make sense to change the value of this on a per-project basis. This is consistent with how none values work for other utilities in the framework.

Some folks in the past have wanted leading-none to be line-height: 0 but technically "leading" is the space between lines, and line-height: 1 removes all extra space between lines so it feels correct to me (although it means all of the numeric utilities like leading-6 are not technically correct but I try hard not to think about that too much).

If someone wants line-height: 0 they can use leading-0 in v4 since the leading-* utilities inherit the spacing scale now.