(#3501) Remove old install migration code

[vexx32]

Description Of Changes

• Remove code relating to migration from a C:\chocolatey install folder from the chocolateysetup.psm1 module used for installing Chocolatey.

Motivation and Context

This migration path hasnt been supported or used since pre-1.0 versions of Chocolatey and is dead code.

Removing this now to simplify the install process somewhat and get the old code out of the way as it's not used at all.

Testing

1. Run build.ps1 to generate a package
2. Copy the generated nupkg to a fresh VM and take a snapshot
3. Install Chocolatey using the following PowerShell code from an admin shell, substituting the nupkg path for the nupkg you copied over:

   ${function:Install-Choco} = [scriptblock]::Create((Invoke-WebRequest https://community.chocolatey.org/install.ps1))
   Install-Choco -ChocolateyDownloadUrl ./path/to/chocolatey.nupkg

4. Ensure the installation completes successfully
5. Downgrade choco to 2.3.0: choco upgrade chocolatey --allow-downgrade --version 2.3.0 --allow-unofficial
6. Upgrade choco once again to the built nupkg to ensure the upgrade works appropriately: choco upgrade chocolatey --source ./path/to/nupkg/folder --pre
7. Ensure the upgrade completes successfully
8. Restore from snapshot
9. Set $env:ChocolateyInstall = 'C:\Users\YOUR_USERNAME\chocolatey'
10. Repeat from step 3 in a non-admin shell to verify things work with a non-admin install as well

Operating Systems Testing

Windows 10

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v3 compatibility checked?

Related Issue

Fixes #3501

[corbob]

Code wise this looks good. I still need to give it a test locally.