More corelib cleanup (dotnet/coreclr#26993)

More corelib cleanup (dotnet/coreclr#26993)

* Make relevant types static (mostly Interop)

* Fix a few stray semicolons

* Remove unnecessary locals

Some can be removed entirely, some can be replaced by discards.

* Remove unnecessary usings

* Move Unix-specific helper method to TimeZoneInfo.Unix.cs

* Remove duplicated code

* Remove duplicative checks

* Remove [ThreadStatic] initialization

These are all the default values and wouldn't be necessary for any static, but inline initialization for [ThreadStatic]s is particularly unuseful (and in non-default cases problematic) because the fields are only set to these values on the initializing thread.

* Fix methods whose return value is never used

* Use string.IsNullOrEmpty

* Remove unnecessary field initialization

All ctors overwrite these values.

* Address PR feedback

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>