chore: cull Splat methods that were never removed

[kentcb]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

chore

What is the current behavior? (You can also link to an open issue here)

Registration methods were duplicated in ReactiveUI and Splat.

What is the new behavior (if this is a feature change)?

Remove the duplicate methods from ReactiveUI.

What might this PR break?

People who explicitly used the RxUI methods. They should just use the Splat ones instead.

Please check if the PR fulfills these requirements

• The commit follows our guidelines: https://github.com/reactiveui/reactiveui#contribute
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[coveralls]

Coverage increased (+0.08%) to 66.75% when pulling 684984b on kentcb:cull-duplicate-ext-methods into f33b96c on reactiveui:develop.

[ghuntley]

This is hangover from when Splat used to be inside of ReactiveUI:

https://reactivex.slack.com/archives/C02AJB872/p1502694681578307