Removes incorrect instructions for SMS that were added in https://github.com/flutter/plugins/pull/6042. These instructions would double-encode the body, causing issues.

This removes, rather than fixes, the instructions because the `sms:` scheme isn't particularly well defined, and has inconsistent handling across platforms (see https://github.com/flutter/flutter/issues/18823 and https://github.com/flutter/flutter/issues/156281 for some examples), so no simple instructions will work everywhere. Handling that kind of platform difference is out of scope for this package and its instructions.

Fixes https://github.com/flutter/flutter/issues/156281