Remove [RegisterSerializer] and support for correspondig Register method

[ReubenBond]

Removes the deprecated & obsolete [RegisterSerializer] attribute and support for the corresponding Register method.

Also fixed the formatting in GrainAttributes.cs since I dislike always having to undo the auto-formatting whenever I make a change.

Fixes #2610

[jdom]

Looks like we implemented option 2 of #2610 (and didn't release it yet, as in wasn't in 1.4.1) but now we are going all the way with option 1. Is my understanding correct?

[ReubenBond]

@jdom the attribute is marked as Obsolete in v1.4.1: https://github.com/dotnet/orleans/blob/v1.4.1/src/Orleans/Core/GrainAttributes.cs#L288

We:

• Obviated the Register method via the [Serializer(...)] attribute (second part of option 1)
• Obsoleted [RegisterSerializer]
• Allowed Register to still work for the time being, but with a necessary modification to the signature < maybe we shouldn't have done this and made the obsoletion message an error instead of a warning.

So now we're doing the first part of option 1.

[jdom]

Sure, that makes sense, I was just saying that this part:

Allowed Register to still work for the time being, but with a necessary modification to the signature < maybe we shouldn't have done this and made the obsoletion message an error instead of a warning.

was actually never released. If you look at the code in v1.4.1, it was still looking for a parameterless Register method. I wonder why we didn't go all the way with option 1, since by looking at the issue, it was agreed in January. Still, looks good, merging, I just wanted to understand

[ReubenBond]

1.4.1 still had static SerializationManager which means registration didn't need to break.