Undo breaking change for OptionsWrapper<T>.Remove(string) and Add(string, TOptions)

[Eilon]

A breaking change was introduced in 2.1 for the methods on OptionsWrapper<T>: Remove(string) and Add(string, TOptions).

• Here's 2.0 with Add/Remove: https://github.com/aspnet/Options/blob/release/2.0/src/Microsoft.Extensions.Options/OptionsWrapper.cs#L25-L38
• Here's a snapshot of dev (2.1) without them: https://github.com/aspnet/Options/blob/b019acac251c24983b93368b06ecfcb3ffbf7604/src/Microsoft.Extensions.Options/OptionsWrapper.cs#L29

This breaking change doesn't seem worthwhile, given how trivial it is to just leave the unused APIs there.

Guidelines on how to obsolete members is here: https://github.com/aspnet/Home/wiki/Engineering-guidelines#breaking-changes

cc @HaoK @ajcvickers

[Eilon]

See discussion here: aspnet/Options#251 (comment)