Removing 'source results only' possibility from PR scan

[eranturgeman]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.
• Update documentation about new features / new supported technologies

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This PR simplifies the auditPullRequestSourceCode func.
We remove the case where we enable the target results to be absent, since in the new flow we only performing a diff scan. Therefore:

1. targetBranchWd cannot be empty unless we had an error in creating its temp dir (in this case the error will be thrown in earlier stage)
2. ResultsToCompare must inhabit a value since if no error happened in the target branch scan we always add these results (if we had an error- it would have thrown in earlier stage)

If both exists as they should - we always add the targetWd path to the scanResultsToIssuesCollection func

[github-actions]

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