Remove the useless function `GetUserIssueStats` and move relevant tests to `indexer_t...

Remove the useless function `GetUserIssueStats` and move relevant tests to `indexer_test.go` (#27067)

Since the issue indexer has been refactored, the issue overview webpage
is built by the `buildIssueOverview` function and underlying
`indexer.Search` function and `GetIssueStats` instead of
`GetUserIssueStats`. So the function is no longer used.
I moved the relevant tests to `indexer_test.go` and since the search
option changed from `IssueOptions` to `SearchOptions`, most of the tests
are useless now.
We need more tests about the db indexer because those tests are highly
connected with the issue overview webpage and now this page has several
bugs.
Any advice about those test cases is appreciated.

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Co-authored-by: CaiCandong <50507092+CaiCandong@users.noreply.github.com>