Drop ProtoSendable.swift since it is no longer needed. #901

Job	Run time
sanitizer_testing (address, debug)	4m 3s
sanitizer_testing (address, release)	6m 14s
sanitizer_testing (thread, debug)	2m 56s
sanitizer_testing (thread, release)	6m 46s
fuzzing_regressions (debug)	1m 54s
fuzzing_regressions (release)	3m 11s
core (5.8.1-jammy, ref/heads/main)	18m 7s
core (5.7.3-jammy, ref/heads/main)	15m 46s
core (5.6.3-focal, ref/heads/main)	11m 43s
	1h 10m 40s

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