remove unused Swift 6 language mode workflow #161

Job	Run time
Compatibility test	5m 10s
Example packages / Windows (nightly-6.0)	2s
Example packages / Windows (6.0)	3s
Example packages / Windows (nightly-main)	3s
Example packages / Linux (5.9)	18m 14s
Example packages / Linux (5.10)	15m 40s
Example packages / Linux (6.0)	17m 54s
Integration test / Linux (5.9)	1m 44s
Example packages / Linux (nightly-6.0)	22m 27s
Integration test / Windows (6.0)	1s
Integration test / Linux (5.10)	1m 34s
Example packages / Linux (nightly-main)	29s
Integration test / Linux (6.0)	1m 47s
Integration test / Linux (nightly-6.0)	2m 12s
Integration test / Windows (nightly-6.0)	1s
Integration test / Linux (nightly-main)	30s
Integration test / Windows (nightly-main)	3s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	1m 41s
Soundness / Format check	36s
Soundness / License headers check	6s
Soundness / Shell check	32s
Soundness / Unacceptable language check	3s
Soundness / Python lint check	5s
Soundness / YAML lint check	0s
Unit tests / Unit tests / Linux (5.9)	6m 23s
Unit tests / Unit tests / Windows (6.0)	4s
Unit tests / Unit tests / Linux (5.10)	1m 43s
Unit tests / Unit tests / Linux (6.0)	2m 0s
Unit tests / Unit tests / Linux (nightly-6.0)	2m 5s
Unit tests / Unit tests / Linux (nightly-main)	32s
Unit tests / Unit tests / Windows (nightly-6.0)	1s
Unit tests / Unit tests / Windows (nightly-main)	2s
	1h 43m 50s

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