remove unused Swift 6 language mode workflow #54

Job	Run time
Integration test / Windows (nightly-6.0)	3s
Integration test / Windows (nightly-main)	2s
Integration test / Linux (5.9)	1m 35s
Soundness / API breakage check	58s
Integration test / Windows (6.0)	3s
Integration test / Linux (5.10)	1m 43s
Integration test / Linux (6.0)	1m 39s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	1m 1s
Integration test / Linux (nightly-6.0)	2m 16s
Soundness / Format check	35s
Integration test / Linux (nightly-main)	32s
Soundness / License headers check	3s
Soundness / Shell check	32s
Soundness / Unacceptable language check	5s
Soundness / YAML lint check	4s
Soundness / Python lint check	4s
Unit tests / Unit tests / Linux (5.9)	59s
Unit tests / Unit tests / Linux (5.10)	50s
Unit tests / Unit tests / Linux (6.0)	55s
Unit tests / Unit tests / Windows (6.0)	3s
Unit tests / Unit tests / Windows (nightly-6.0)	3s
Unit tests / Unit tests / Linux (nightly-6.0)	1m 15s
Unit tests / Unit tests / Windows (nightly-main)	3s
Unit tests / Unit tests / Linux (nightly-main)	30s
	15m 56s

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