Remove `concrete::PolicyArc` #168

Job	Run time
fuzz (roundtrip_miniscript_str)	5m 7s
fuzz (roundtrip_miniscript_script)	4m 35s
fuzz (parse_descriptor)	3m 49s
fuzz (roundtrip_semantic)	3m 42s
fuzz (parse_descriptor_secret)	3m 45s
fuzz (roundtrip_descriptor)	3m 58s
fuzz (roundtrip_concrete)	3m 51s
fuzz (compile_descriptor)	3m 45s
verify-execution	0s
	32m 32s

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