Remove start page code

[kimadeline]

In #16417 we made a small change to the start page code (see this comment thread), but a general review of startPage.ts and startPage.tsx might be needed.

A couple of things I noticed here and there, but by no means an exhaustive list:

• Why are openFolder and openWorkspace defined differently in
  https://github.com/microsoft/vscode-python/blob/main/src/startPage-ui/startPage/startPage.tsx#L170-L180
• Why are some of the open____ methods public, and others are private?
• When having the start page open, clicking on "Open a Folder" or the folder icon or the "Open a Folder or Workspace" header doesn't seem to do anything

Do this work as part of #16678

[DonJayamanne]

Moving back to Python repo, as this start page applies to Python all up, not just DS.

[karthiknadig]

Since we are moving to VS Code start pages, we will be removing this.