fix flaky test TestDiscardedSeriesTracker

[SungJin1212]

I have found the test TestDiscardedSeriesTracker is flaky; the logs are:

--- FAIL: TestDiscardedSeriesTracker (0.00s)
    tracker_test.go:114: 
        	Error Trace:	/__w/cortex/cortex/pkg/util/discardedseries/tracker_test.go:114
        	            				/__w/cortex/cortex/pkg/util/discardedseries/tracker_test.go:81
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 1
        	Test:       	TestDiscardedSeriesTracker
FAIL

How to reproduce

go test -count=100 -race -run TestDiscardedSeriesTracker ./pkg/util/discardedseries

Like #7036, the usersToDelete should be re-initialized for each loop.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]