Simplify remote URL handling in EnableGitRemoteAccess

[JayBazuzi]

• don't create a Uri object, as UriBuilder will accept a string
• don't ToString() the result, as it happens automatically
• use falsyness to check that $Env:SYSTEM_ACCESSTOKEN is set.
• skip an uneeded local ($token)

[pascalberger]

@JayBazuzi Thank you very much for the PR. Do you think you can have a look at the above comments before we can merge this?

[pascalberger]

I merged this and will fix the open comments in another PR here: https://github.com/iozag/vsts-git-tasks/pull/10