Remove encoding workaround

[MichalStrehovsky]

\src\System.Private.CoreLib\src\System\Environment.EnvironmentVariables.Unix.cs has a workaround to return null for LANG environment variable. This is to prevent System.Console from taking complicated code paths that we don't support yet.

The required feature is either interface calls or constrained calls (don't remember which one...).

[jkotas]

This workaround was moved to src\System.Private.CoreLib\src\System\Text\Encoding.cs

[MichalStrehovsky]

Fixed by #1732.

[MichalStrehovsky]

Only 50% fixed :)

[MichalStrehovsky]

Fixed by #1971.