Remove the experimental Utf8String type and tests from the runtime repo

[trylek]

I'm proposing to remove the Utf8String and Utf8Span types and the
System.Utf8String.Experimental tests from the runtime repo as according to
JanK's clarifications these types and tests are currently treated as experimental
and belong to the runtimelab, not to the "production" runtime repo. From a
practical perspective, we're hitting crashes in the Utf8String tests when using
System.Private.CoreLib compiled with Crossgen2 which doesn't yet implement
the Utf8String-related typesystem logic.

Thanks

Tomas

Related to: #41521

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Issue meta data

Issue content:	I'm proposing to remove the Utf8String and Utf8Span types and the System.Utf8String.Experimental tests from the runtime repo as according to JanK's clarifications these types and tests are currently treated as experimental and belong to the runtimelab, not to the "production" runtime repo. From a practical perspective, we're hitting crashes in the Utf8String tests when using System.Private.CoreLib compiled with Crossgen2 which doesn't yet implement the Utf8String-related typesystem logic. Thanks Tomas Related to: #41521
Issue author:	trylek
Assignees:	-
Milestone:	[object Object]

[GrabYourPitchforks]

Don't forget to remove the file runtime/src/libraries/System.Private.CoreLib/src/System/Char8.cs and all references to the Char8 type.

If you search the repo's .cs files for FEATURE_UTF8STRING, you'll find some lingering references in Utf8Utility.cs, Span.cs, Memory.cs, and related files. Those can all be removed as well.

[trylek]

Merging now to unblock my follow-up Crossgen2 efforts. I'll be happy to address any additional feedback in a follow-up PR.