Remove outdated README reference from transports.md #2347

Job	Run time
build (ubuntu-latest, Release)	7m 14s
build (windows-latest, Release)	10m 40s
build (macos-latest, Release)	4m 48s
build (ubuntu-latest, Debug)	6m 39s
build (windows-latest, Debug)	11m 59s
build (macos-latest, Debug)	5m 40s
publish-coverage / publish-coverage	53s
	47m 53s

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