[css-properties-values-api] Remove <transform-list>?

[andruud]

Does it make sense to keep this type around when <transform-function>+ is allowed?

[andruud]

OK, I now realize that <transform-list> probably exists because of the special interpolation rules [1].

[1] https://www.w3.org/TR/css-transforms-1/#interpolation-of-transforms