Cleanup contents of util

[donny-dont]

A number of functions in util.dart aren't used now. Those are now removed. Formatted the file accordingly.

writeStreamToSink and isPlainAscii moved into the multipart implementation at 2097129

[nex3]

I'm not sure I see the logic behind moving writeStreamToSink() and isPlainAscii() out of the utils directory. They seem like pretty generic utilities to me.

[donny-dont]

@nex3 they weren't being used by anything outside of MultipartBody. Also wasn't sure if there was something in async that might be a replacement for writeStreamToSink.

[nex3]

they weren't being used by anything outside of MultipartBody.

I think a better criterion for utils is "Stuff whose use is not specific to the place(s) where it's used", rather than "Stuff that's used in multiple places".

Also wasn't sure if there was something in async that might be a replacement for writeStreamToSink.

I don't know of anything that specifically writes without closing the sink.