Remove unnecessary calls to Contains for sets

[mpidash]

This commit removes calls to Contains if they are immediately followed by either Add or Remove.
These calls were found during testing of a new analyzer (CA1865), see dotnet/roslyn-analyzers#6767 and dotnet/runtime#85490.

[AlekseyTs]

Done with review pass (commit 1)

[mavasani]

This commit removes calls to Contains if they are immediately followed by either Add or Remove.
These calls were found during testing of a new analyzer (CA1865), see dotnet/roslyn-analyzers#6767 and dotnet/runtime#85490.

I believe you should also set CA1865 to be a build warning in the editorconfig at the repo root to prevent more violations sneaking in future.

[mpidash]

I believe you should also set CA1865 to be a build warning in the editorconfig at the repo root to prevent more violations sneaking in future.

Should I add it for all [*.{cs,vb}]? That would be the first explicit rule there.
A few other rules are set for [src/{Analyzers,CodeStyle,Features,Workspaces,EditorFeatures,VisualStudio}/**/*.{cs,vb}].

[AlekseyTs]

Changes under Compilers LGTM (commit 2)

[sharwell]

I believe you should also set CA1865 to be a build warning in the editorconfig at the repo root to prevent more violations sneaking in future.

It should be in .globalconfig, not .editorconfig.
https://github.com/dotnet/roslyn/blob/main/eng/config/globalconfigs/Common.globalconfig