Remove `disable_combine_sarif_files` feature flag #4538

Job	Run time
Diagnostic export (ubuntu-latest, linked)	1m 16s
Diagnostic export (windows-latest, nightly-latest)	3m 39s
Diagnostic export (macos-latest, linked)	2m 8s
Diagnostic export (macos-latest, nightly-latest)	1m 55s
Diagnostic export (windows-latest, linked)	4m 59s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 21s
	15m 18s

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