Drop ProtoSendable.swift since it is no longer needed. #903

Job	Run time
core (5.8.1-jammy, ref/heads/main)	14m 30s
core (5.7.3-jammy, ref/heads/main)	12m 36s
core (5.6.3-focal, ref/heads/main)	12m 12s
sanitizer_testing (address, debug)	3m 3s
sanitizer_testing (address, release)	6m 43s
sanitizer_testing (thread, debug)	3m 2s
sanitizer_testing (thread, release)	7m 9s
fuzzing_regressions (debug)	2m 15s
fuzzing_regressions (release)	3m 24s
	1h 4m 54s

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