Fix CA1805 for unnecessary initialization of fields

[stephentoub]

This has three primary benefits:

1. In some cases, in particular for static fields where other static fields have non-default initialization, the C# compiler can't elide the default initialization, resulting in more IL.
2. It highlights places where the field is otherwise unused and can be deleted, but where the presence of the initialization was silencing the C# compiler's warnings about the unused field.
3. It increases consistency and decreases maintenance / code size / etc.

cc: @marek-safar, @bartonjs

[danmoseley]

Are you going to enable the rule so we don't go backwards (just waiting on that Roslyn fix?)

Did the rule have a fixer?

[stephentoub]

Are you going to enable the rule so we don't go backwards

Eventually. There are a few cases I reverted because they require more follow-up, e.g. #38401, generally where removing the initialization then caused C# to warn that the field was unused, and while I deleted a few, I didn't go through all of them.

Did the rule have a fixer?

Yes, I wrote a fixer for it and used that to do this:
https://github.com/dotnet/roslyn-analyzers/blob/master/src/NetAnalyzers/CSharp/Microsoft.CodeQuality.Analyzers/QualityGuidelines/CSharpDoNotInitializeUnnecessarily.Fixer.cs

[bartonjs]

Wow, that was a lot of "Load Diff" clicking.

[danmoseley]

Looks good except I don't follow HeaderCollection.cs

I did not review inside Github :)