Remove invalid gradient fallbacks

[adamwathan]

Prior to this PR we were providing fallback values for certain CSS variables in our gradient utilities that just weren't necessary and didn't do anything.

For example bg-linear-to-r was generating this:

.bg-linear-to-r {
  --tw-gradient-position: to right;
  background-image: linear-gradient(
    var(--tw-gradient-stops, to right)
  );
}

…but background-image: linear-gradient(to right) is not valid CSS and is thrown out by the browser.

This PR removes these fallback values entirely since there is nothing sensible to fall back to anyways — you need to combine these utilities with the from-*/to-* utilities or provide the complete gradient as an arbitrary value for things to make sense.