PMM-12547 Revert PMM-11670 Set the cluster label if empty (#2508) #9515

Job	Run time
Tests (mysql:5.6, mongo:4.2, postgres:10, percona/pmm-server:2.0.0)	1m 26s
Tests (mysql:5.7, mongo:4.4, postgres:11, percona/pmm-server:2.0.1)	1m 23s
Tests (mysql:8.0, mongo:4.4, postgres:12, percona/pmm-server:2.1.0)	2m 16s
Tests (mysql:8.0, mongo:4.4, postgres:13, percona/pmm-server:2.2.0)	1m 13s
Tests (percona:5.6, percona/percona-server-mongodb:4.2, perconalab/percona-distribution-postgresq...	1m 12s
Tests (percona:5.7, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	2m 48s
Tests (percona:8.0, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	1m 10s
Tests (mariadb:10.2, mongo:4.4, postgres:9.4, perconalab/pmm-server:dev-latest)	1m 10s
Tests (mariadb:10.3, percona/percona-server-mongodb:4.4, postgres:9.5, perconalab/pmm-server:dev-...	1m 28s
Tests (mariadb:10.4, postgres:9.6, perconalab/pmm-server:dev-latest)	58s
	15m 4s

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