Remove windows Batch from shell executor

Update: May 19 2020

Based on customer feedback, we have decided that we will NOT be removing Windows Batch (Cmd) from the Windows executor with a few caveats:

1. PowerShell is going to be the default shell, even if shell is not defined. Users will have to specify shell = "cmd" if they want to use Windows batch.
2. We will no longer maintain cmd meaning:
   • No bug fixes
   • No support
   • No new features will be added.
3. We will not mention Windows batch in our documentation apart from https://gitlab.com/gitlab-org/gitlab-runner/-/blob/eddf58f38b6f0c7de5fbf94f69165d1c71b4b550/docs/shells/index.md#L72

Overview (Prior to May 19 2020)

In #4163 (closed) we deprecated Windows Batch cmd for the shell executor. Remove anything related to the cmd script generation. If the user-defined something else other then bash or powershell we should show an error message to the user.