Cleanup updatablehelpsystem and enable XSD validation on MAML help content

[iSazonov]

PR Summary

Related #3565 and #4357.

Easily review commit by commit:

• Remove old code for WebClient - currently HTTPClient is main in CoreFX.
• Restore XML Schema support - currently we have the API in CoreFX.
• Reorder using statements.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.
• User-facing changes
  • Not Applicable
  • OR
  • User-facing Documentation needed
    • Issue filed - Issue link:
• Testing - New and feature
  • Not Applicable or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
    • Add [feature] if the change is significant or affects feature tests

[iSazonov]

Tests failed with "The Help content XSD file cannot be found in C:\projects\powershell\src\powershell-win-core\bin\Release"

I see that Windows PowerShell has Schemas directory in home folder.

Should we restore this or remove schemas support?

[adityapatwardhan]

@iSazonov Thanks for this PR. We should bring back the schema files as XSD validation seems to be supported in CoreCLR now.

[iSazonov]

@adityapatwardhan Please point me where I can get this files from. Can I copy them to the repo from Windows PowerShell directory?

[iSazonov]

@adityapatwardhan Schemas added.

[adityapatwardhan]

Please update the title to mention enable XSD validation of maml help