Remove `disable_combine_sarif_files` feature flag #4509

Job	Run time
Diagnostic export (macos-latest, linked)	2m 11s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 21s
Diagnostic export (windows-latest, linked)	4m 0s
Diagnostic export (ubuntu-latest, linked)	1m 4s
Diagnostic export (macos-latest, nightly-latest)	2m 19s
Diagnostic export (windows-latest, nightly-latest)	3m 30s
	14m 25s

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