remove mantle_backup_duration_seconds_total metric #828

Job	Run time
build (3Mi, change-to-primary)	5s
build (3Mi, full-backup)	7s
build (3Mi, change-to-secondary)	9s
build (3Mi, incr-backup)	1s
build (3Mi, change-to-standalone)	2s
build (3Mi, !//)	1s
build (10Mi, various-transfer-part-size)	1s
build (3Mi, backup-failure)	1s
build (200Gi, various-transfer-part-size)	1s
	28s

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