Remove discontinued install-dotnet-preview.sh

[MichaelSimons]

Per the discussion at #635 (comment), a decision was made to discontinue this file as detectable usage was pretty close to 0. The appropriate deprecation steps should be taken and the file should be removed.

[ellahathaway]

We should revert the changes in #637 as part of this work.

[ellahathaway]

Completed in #648