Stop syncing value attribute for controlled inputs

Opening this as a follow up to some quick discussions in https://github.com/facebook/react/issues/11881. Syncing the `value` attribute has been a consistent source of bugs for us, and the benefits of doing so seem minimal. There's some previous discussion on the topic in https://github.com/facebook/react/pull/7359 and in other issues, I can't remember right now 😄 This would be a breaking change, so it would have to be done in a major release. ## Reasons to keep syncing * It prevents `form.reset()` from putting controlled form inputs into a weird state * Some browser extensions (not sure which) read from the `value` attribute in some cases (not sure which) * It can be useful for querying inputs with a specific value using an attribute selector ## Reasons to stop syncing * It will reduce the complexity of `react-dom` in a non-trivial way * In turn, it will likely reduce bundle size as well * We remove a whole class of bugs (fighting with browsers that want to be helpful about input values) * Syncing the input value to the attribute potentially exposes sensitive data to third party tools ([1](https://www.reddit.com/r/analytics/comments/7ukw4n/mixpanel_js_library_has_been_harvesting_passwords/)) ______ What do we think? Are these reasons good enough to keep syncing the `value` attribute? Are there other more critical reasons we should keep doing so? cc @nhunzaker @jquense @gaearon