Remove pattern string constructor from rust_icu_udat

ECMA-402 does not and [probably](https://github.com/tc39/ecma402/issues/342) [will](https://github.com/tc39/ecma402/issues/147) [not](https://github.com/tc39/ecma402/issues/1) add support for parsing. The bottom line is that parsing of localized strings is a hard problem that is best solved by simply building your application to avoid having to do it. Do you really need to support parsing in the wrapper?

ECMA-402 also does and probably will not support patterns. ICU has long recommended that people don't use patterns; they should use skeletons instead. Do you really need to support patterns?

Speaking of skeletons, you don't have an API for skeletons right now. Could you remove your pattern API and replace it with a skeleton API?