Remove unused settings and disable updating to prereleases of the CodeQL CLI by default

[henrymercer]

Remove ql.distribution.owner and ql.distribution.repository settings from the workspace since they are unused, and remove the ql.distribution.includePrereleases setting to disable including prereleases by default.

[jcreedcmu]

Do we want to references remove these from src/{distribution,config}.ts as well? I can't recall of the top of my head if vscode breaks or merely returns undefined if you try to ask for configuration items that aren't in package.json.

[henrymercer]

I've just checked and you're right that includePrerelease is still referenced in src/{distribution,config}.ts. The owner and repository settings haven't been referenced since github/vscode-codeql#174. The behaviour of VS Code is to return undefined when you look up a configuration item that isn't in package.json, so having an includePrelease setting that isn't in the package.json doesn't break anything.

I've updated the title of this PR and the commit message accordingly.

[jcreedcmu]

lgtm in that case